he  Photography 
of  Coloured 
Objects 

* 


: - 


Second  Edition 


■'"v 


Eastman  Kodak  Co. 

Rochester,  N.Y. 
1916 


V 


The  Photography 
of  Coloured 
Objects 

* 


Second  Edition 


Eastman  Kodak  Co. 

Rochester,  N.Y. 

1 9 1 6 


First  Edition,  1909. 
Reprinted,  1913. 

Second  Edition,  1916. 


PREFACE 


rFNHE  first  edition  of  “ The  Photography  of  Coloured 
1 Objects  ” was  written  by  Dodtor  C.  E.  Kenneth 
Mees,  who  stated  it  was  an  attempt  to  put  clearly  the 
theory  underlying  the  photography  of  coloured  objects 
and  the  application  of  that  theory  to  those  branches  of 
practice  which  are  of  the  most  immediate  importance. 

This  revision  includes  most  ot  the  matter  contained 
in  the  previous  edition,  together  with  an  incorporation 
of  much  of  that  contained  in  the  Wratten  book  on 
“ Orthochromatic  Filters  ” which  was  also  written  by 
Dr.  Mees. 

While  purely  scientific  nomenclature  and  phraseology 
are  not  employed,  no  attempt  has  been  made  to  be  en- 
tirely u practical,”  since  the  application  of  an  ounce  of 
accurate  knowledge  may  be  worth  a ton  of  unreasoning 
practice.  No  pretence  is  made  of  being  unbiassed,  though 
it  is  hoped  that  there  is  no  conscious  bias.  The  Wratten 
products  are  freely  discussed,  but  the  loss  of  generaliza- 
tion caused  by  this  procedure  will  be  compensated  by 
the  advantage  to  be  gained  from  definite  information. 

The  large  number  of  friends  who  have  kindly  assisted 
in  the  compilation  of  this  volume  makes  it  impossible 
to  acknowledge  all  by  name,  and  it  would  perhaps  be 
invidious  to  mention  only  a few.  We  hope  that  all  will 
understand  that  although  they  are  not  named  we  are 
none  the  less  grateful  to  them  for  their  various  sugges- 
tions, and  that  we  shall  also  be  grateful  for  suggestions 
for  the  improvement  of  future  editions. 

EASTMAN  KODAK  CO. 

Rochester,  N.Y. 


■ 


CONTENTS 

CHAP.  PAGE 

Preface  ........  3 

I.  The  Nature  of  Colour  .....  7 

II.  The  Sensitiveness  to  Coloured  Light  of  the 

Eye  and  of  Photographic  Plates  . . 19 

III.  Orthochromatic  Filters  ....  24 

IV.  The  Efficiency  of  Orthochromatic  Filters  31 

V.  The  Multiplying  Factor  of  any  Sharp-Cut 

Filter  .......  39 

VI.  The  Rendering  of  Colour  Contrasts  . . 47 

VII.  Portraiture  . . . . . . .61 

VIII.  Landscape  Photography  ....  69 

IX.  The  Photography  of  Coloured  Objects  for 

Reproduction  ......  77 

X.  Three-Colour  Photography  ....  82 

XI.  The  Optical  Properties  of  Filters  . . 97 

XII.  The  Fitting  of  Filters  . . . .105 

XIII.  The  Care  of  Filters  . . . . .112 

Index  . . . . . . . 1 1 5 


5 


THE  PHOTOGRAPHY  OF 
COLOURED  OBJECTS 

CHAPTER  I 

THE  NATURE  OF  COLOUR 

\T  the  commencement  of  this  book,  which  is  essentially 
concerned  with  the  analysis  and  photography  of  colour, 
it  will  be  well  for  us  to  get  a definite  idea  as  to  what  is 
meant  by  “colour,”  and  with  what  physical  phenomena  colour 
is  associated. 

The  nature  of  colour  is  involved  in  the  conception  we  obtain 
as  to  the  nature  of  light.  The  nature  of  light  has  long  been  a 
source  of  speculation,  and  it  was  generally  held  that  perception 
of  light  depended  on  the  reception  by  the  eye  of  small  discrete 
particles  shot  off  from  the  source  of  light ; just  as  at  one  time  it 
was  held  that  the  perception  of  sound  depended  upon  the  impact 
upon  the  ear  drum  of  small  particles  shot  oft'  from  the  sources 
of  the  sound.  This  theory  of  light  has  the  advantage  that  it 
immediately  explains  reflection  ; just  as  an  indiarubber  ball 
bounces  from  a smooth  wall,  while  it  will  be  shot  in  almost  any 
direction  by  a heap  of  stones,  so  these  small  particles  would 
rebound  from  a polished  surface,  while  a rough  surface  would 
merely  scatter  them.  This  theory  of  the  nature  of  light  appeared 
adequate  until  it  was  found  that  it  was  possible,  by  dividing  a 
beam  of  light  and  slightly  lengthening  the  path  of  one  of  the 
halves,  and  then  re-uniting  them,  to  produce  periods  of  darkness 
similar  in  nature  to  the  nodes  produced  in  an  organ-pipe,  where 
the  interference  of  waves  of  sound  is  taking  place.  It  could  not 
be  imagined  that  a reinforcement  of  one  stream  of  particles  by 
another  stream  of  particles  in  the  same  dire&ion  could  produce 


/ 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

an  absence  of  particles,  while  the  analogy  with  sound  suggested 
that,  just  as  sound  was  known  to  consist  of  waves  in  the  air,  so 
light  also  consisted  of  waves. 

Light  cannot  consist  of  waves  in  the  air,  partly  because  we 
know  that  it  travels  through  interstellar  space,  where  we  imagine 
that  there  is  no  air,  but  also  because  the  velocity  of  light,  nearly 
200,000  miles  per  second,  is  so  great  that  it  is  impossible  that  it 
could  consist  of  a wave  in  any  material  substance  with  which 
we  are  acquainted.  It  is,  however,  supposed  that  there  must 
exist,  spread  through  all  space  and  all  matter,  a substance  which 
is  termed  the  ether,  and  that  light  consists  of  waves  in  this 
ether. 

Now,  just  as  in  sound  we  have  wave  notes  of  high  frequency, 
that  is,  with  many  waves  per  second  falling  upon  the  ear,  which 
form  the  high-pitched  or  shrill  notes,  and  also  notes  of  low 
frequency,  where  only  a few  waves  per  second  fall  upon  the 
ear,  forming  the  bass  notes — so  with  light  we  may  have  different 
frequencies  of  vibration,  some  falling  upon  the  eye  at  very  short 
intervals,  while  other  waves  are  of  only  half  or  even  less  fre- 
quency. 

Since  the  velocity  of  light  is  the  same  for  waves  of  different 
frequencies,  it  is  clear  that  the  waves  of  high  frequency  will  be 
of  shorter  wave  length  than  those  of  low  frequency,  the  length 
of  a light  wave  being  the  distance  from  the  crest  of  one  wave  to 
the  crest  of  the  next. 

The  wave  length  of  the  light,  like  the  velocity,  varies  with 
the  medium  in  which  the  light  is  travelling.  For  instance, 
when  light  is  travelling  through  glass,  it  will  only  have  about 
two-thirds  of  the  wave  length  of  the  light  travelling  in  the  air. 
But  it  is  convenient  to  consider  simply  the  wave  length  of  light 
as  the  length  of  the  wave  in  free  ether,  or  for  pra&ical  purposes, 
in  air.  White  light  consists  of  vibrations  of  many  degrees  of 
frequency,  i.e.y  it  consists  of  waves  of  various  lengths;  and  a 
mixture  of  waves  of  all  lengths  in  certain  proportions  forms 
what  we  term  white  light.  If  instead  of  allowing  this  hetero- 
geneous mixture  of  waves  to  fall  upon  the  eye,  we  omit  waves 
of  some  frequencies  from  those  entering  the  eye,  then  the  brain 
will  receive  a sensation  of  colour.  Thus  colour  is  associated 
with  wave  length.  White  light  being  made  up  of  waves  of 
different  lengths  may  be  regarded  as  being  made  up  of  light  of 
various  colours,  and  by  different  devices  may  be  split  up  into 
these  colours. 

We  can  analyse  white  light  or  discover  the  composition  of 
any  light  with  the  spectroscope,  an  instrument  by  means  of 

8 


THE  NATURE  OF  COLOUR 


RED 


GREEN 


which  a small  portion  of  the  light  to  be  examined  is  passed 
through  a prism,  or  transmitted  by,  or  reflected  from  a diffrac- 
tion grating.  The  result  is  that 

the  light  is  split  up  into  a band  WAVE  LENGTHS 
of  different  colours,  which  is 
known  as  the  spedtrum.  If  the 
light  analysed  is  white  these 
colours  merge  into  one  another 
without  any  break,  but  there 
will  be  a break  or  breaks  (ab- 
sorption bands)  if  the  light  ex- 
amined is  coloured.  Figure  I 
shows  the  relative  length  of 
the  waves  corresponding  with 
light  of  various  colours,  the 
diagram  being  drawn  to  scale. 

Since  different  length  of  waves 
correspond  with  different  col- 
ours, a scale  mav  be  made  in 
which  thedifferent  wave-length 

numbers  represented  corre-  BLUE 

spond  in  position  with  the  dif- 
ferent colours  of  rhespedtrum. 

The  following  diagram  gives  a 
simple  arrangement  of  the  nor- 
mal spedtrum,  the  numbers  re-  pIG  , 

presenting  the  length  of  the 

waves  in  Angstrom  Units  (A.U.),  which  are  ten-millionths  of 
millimetres,  and  the  colours  being  placed  against  them  : 


BLUE  VIOLET 

1 

BLUE 

GREEN 

1 

GREEN 

T 

ORANCf 

rauiv 

1 

RED 

4000 

5000 

6000 

7000 

Fig.  2 


It  will  be  seen  that  the  visible  spedtrum  extends  from  4,000 
to  7,000,  and  is  equally  divided  into  regions  which  may  be 
broadly  termed  : 


Blue-violet 

Green 

Red 


4.000  to  5,000 

5.000  „ 6,000 

6.000  ,,  7,000 


9 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

Light-filters,  that  is  transparent  media  absorbing  certain 
waves  and  transmitting  others,  can  be  constructed  which  will 
absorb  some  particular  region  of  the  speCtrum,  and  they  are 
generally  called  after  the  colour  they  transmit;  thus  if  we  make 
a filter  which  only  lets  through  the  portion  of  the  speCtrum 
between  4,000  and  5,000,  then  we  should  call  that  filter  a blue- 
violet  filter,  a filter  letting  through  from  5,00c  to  6,000  would 
be  a green  filter,  and  a filter  letting  through  from  6,000  to  7,000 
would  be  red  in  colour.  Thus  from  the  spe&rum  we  already 
derive  the  idea  that  light  can  be  divided  into  three  colours 
which  we  may  call  the  primary  colours,  red,  green,  and  blue- 
violet. 

Remembering  this  conception  of  light,  let  us  consider  why 
we  term  a given  filter  red.  It  will  appear  red  because  it  only 
lets  through  red  light,  but  white  light  consisting  of  blue-violet, 
green,  and  red  is  falling  upon  it,  so  that  clearly  it  is  red  because 
it  stops  or  absorbs  the  blue-violet  and  green  light. 

Similarly,  a piece  of  red  paper  is  red  because  it  reflects  red 
light,  but  it  has  falling  upon  it  white  light  consisting  of  blue- 
violet,  green,  and  red,  so  that  it  must  absorb  the  blue-violet  and 
green  light,  not  reflecting  them,  but  only  reflecting  the  red  light. 
We  are  therefore  justified  in  saying  that  anything  which  absorbs 
blue-violet  light  and  green  light  together  will  be  red. 

It  is  this  aspeCt  of  colour,  that  objeCts  are  coloured  because 
they  absorb,  which  must  be  clearly  and  definitely  understood  if 
the  best  results  are  to  be  obtained  in  the  photography  of  coloured 
objeCfs.  Unfortunately,  however,  the  conception  of  colour  as 
an  absorption  is  not  common,  though  it  is  the  most  useful  one, 
and  it  will  be  necessary  somewhat  to  elaborate  this  idea  in 
order  to  prevent  misconceptions  arising.  We  should  form  the 
habit  of  considering  a red  objeCt,  not  as  one  that  refleCts  red, 
but  as  one  that  absorbs  blue-violet  and  green. 

The  importance  of  this  definition  is  that  it  defines  “red” 
without  reference  to  the  colour  of  the  incident  light,  l ake  a 
scarlet  book  and  examine  it  by  a light  containing  no  red;  such 
for  instance  as  the  mercury  vapour  lamp,  in  which  red  is  almost 
entirely  wanting.  The  book  will  no  longer  refleCt  red  light 
because  there  is  no  red  light  for  it  to  refleCl,  but  it  will  still 
absorb  the  blue-violet  and  green  light  of  the  lamp,  and  will 
look  black ; it  has  not,  of  course,  changed  its  nature,  and  we 
should  still  be  justified  in  saying  that  it  is  red  if  we  define  red 
as  we  have  done  above. 

In  the  same  way  a yellow  objeCt  is  not  one  which  refleCls 
yellow  light  (there  is  very  little  yellow  light  indeed  in  the 

10 


THE  NATURE  OF  COLOUR 


spedlrum,  and  if  an  objedt  refledted  only  yellow  light  it  would 
be  so  dark  as  to  be  almost  black),  but  a yellow  colour  is  due  to 
blue  absorption.  It  reflects  the  other  two  components  of  white 
light,  green  and  red,  so  that  we  should  be  justified  in  saying  that 
yellow  light  consists  of  green  light  plus  red  light,  but  for  our 
purpose  let  us  consider  yellow  simply  as  a lack  of  blue;  yellow 
is  minus  blue,  so  that  if  you  have  a beam  of  yellow  light  and 
add  blue  light  to  it,  you  will  get  white  light. 

Now  what  is  green?  Well,  since  white  light  consists  of  blue 
light,  green  light,  and  red  light,  green  is  clearly  white  light 
minus  red  and  minus  blue;  and  a green  body  is  one  which 


BLUE 

GREEN 

RED 

t|BLUEt| 

YE  LI 
GREEN 

-OW 

RED 

||blue|| 

|green| 

RED 

■MSI 

||BLUE% 

GREEN 

fc|RED 

Fig.  3 

absorbs  both  red  and  blue.  The  difference  between  a green 
objedt  and  a yellow  objedt  being  that  the  yellow  objedt  absorbs 
blue  only,  whereas  the  green  object  also  absorbs  most  of  the  red 
light  which  the  yellow  objedt  refledts. 

We  can  now  make  clear  what  is  meant  by  complementary 
colours.  As  is  shown  in  the  diagram  (fig.  3),  white  light  con- 
sists of  blue  light,  green  light,  and  red  light.  The  next  sedtion 
under  this  shows  the  blue  blotted  out,  leaving  the  mixture  of 
green  and  red — that  is,  yellow.  We  should  say,  then,  that  yellow 
is  complementary  to  the  blue-violet.  In  the  same  way,  in  the 
next  diagram  all  blue  and  green  are  blotted  out,  leaving  only 
red,  so  that  red  is  complementary  to  blue-green.  In  the  bottom 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

diagram  all  blue  and  red  are  blotted  out,  leaving  only  green  ; 
green  is  complementary  to  this  blue-red  mixture,  which  is  usually 
known  as  magenta. 

In  general,  then,  the  light  absorbed  by  an  object  may  be  said 
to  be  complementary  to  that  reflected  by  it. 

So  far  we  have  only  considered  intense  colours.  We  have 


imagined  that  a red  objedl  absorbs  the  whole  of  the  blue-violet 
and  the  green  light,  that  is  to  say  that  its  absorption  was  com- 
plete. But  most  things  have  only  partial  absorption — the  absorp- 
tion is  incomplete.  Partial  absorption  can  be  of  two  forms:  it 
can  be  gradual,  or  it  can  be  sharp;  thus,  if  when  taking  a photo- 
graph of  a spedlrum  there  is  put  in  front  of  the  spedtroscope 


Fig.  5.  Gentian  Violet  Absorption  Spectrum 


a solution  of  erythrosine  then  that  erythrosine  will  absorb  a clean 
patch  of  green  from  the  spedtrum  between  4,800  and  5,500,  as 
is  shown  in  the  photograph  (fig.  4).  But  if  you  put  in  front  of 
the  spedtroscope  slit  a cell  containing  gentian  violet  you  will 
get  a very  gradual  diminution  of  intensity  between  about  4,700 
and  6,200,  with  the  least  light  transmitted  about  5,800  (fig.  5). 
Thus  different  dyes  and  different  substances  give  different  classes 
of  absorption,  the  two  kinds  being  roughly  subdivided  into 
(1)  sharp  absorptions,  and  (2)  gradual  absorptions. 

12 


THE  NATURE  OF  COLOUR 


Let  us  examine  the  effect  of  a single  sharp  absorption  band  in 
different  parts  of  the  s-pedtrum.  First,  consider  a sharp  absorp- 
tion band  situated  in  the  red  about  6,500  and  producing  a total 
absence  of  red  in  this  part.  The  remaining  colour  consists  of  all 
the  blue-violet  and  all  the  green,  with  some  of  the  red.  The 
actual  visual  effedt  of  the  mixed  colour  is  what  one  might  term 
a “sky-blue.”  Imagine  this  band  to  shift  so  as  to  absorb  the 


CHART  SHOWING  RESIDUALS 

POSITION  OF  ABSORPTION  BANDS 


RED  GREEN  BLUE 

Fig.  6 


orange;  absorbing  between  5,800  and  6,200,  the  colour  will  be 
a light  violet-blue,  because  there  is  a great  deal  of  red  being 
transmitted  and  less  green.  If  the  band  shifts  into  the  yellowish 
green  from  5,600  and  6,000  it  will  absorb  a great  deal  of  the 
green  and  none  of  the  red,  and  the  colour  will  become  bluish 
purple;  as  it  shifts  lower  in  the  green  towards  the  blue  this 
purple  becomes  a reddish  purple,  so  that  when  the  band  is 
situated  at  from  5,600  to  5,200  we  have  what  is  generally  known 
as  magenta  in  colour.  As  the  band  shifts  towards  the  blue,  the 
blue  fades  out  of  the  magenta,  green  taking  its  place.  When 

*3 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

the  band  is  from  4,700  to  5,200  the  colour  is  a sort  of  orange, 
and  as  the  band  moves  into  the  blue-violet  the  orange  becomes 
a yellow,  and  finally  a lemon-yellow.  So  that  if  we  imagine  a 
single  band  to  pass  down  the  spedtrum,  we  get  a change  from 
light  sky-blue  through  purple,  magenta,  orange,  and  yellow,  to 
lemon-yellow  (fig.  6). 

Now  it  will  be  seen  that  there  is  one  class  of  colour  which 
does  not  enter  at  all  into  this  series,  namely,  the  greens.  There 
is  really  no  visual  suggestion  of  green  in  any  colour  formed  by 
using  a daylight  spedtrum  and  absorbing  one  narrow  band  only. 
In  order  to  get  a green  we  must  have  an  absorption  both  in  the 
red  and  in  the  blue.  If  we  absorb  the  extreme  blue  and  also 
the  extreme  red,  we  shall  at  once  get  a green,  and  as  these  two 
bands  vary  with  regard  to  each  other,  we  shall  obtain  various 
shades  of  greens.  'Thus,  if  the  blue  absorption  band  is  weak, 
and  the  red  absorption  band  is  very  strong,  we  get  blue-greens ; 
if  the  red  absorption  weak,  and  the  blue  strong,  yellow-greens. 

Green  is  almost  the  only  common  colour  due  to  two  absorp- 
tion bands,  and  other  colours  which  on  analysis  prove  to  have 
two  absorption  bands  generally  tend  to  be  mere  variants  in  hue 
of  some  colours  which  we  have  already  discussed  under  the 
heading  of  single  absorption  bands.  A brown  colour  is  fairly 
common,  and  the  bands  of  a brown  are  of  a gradual  absorption 
type  generally  extending  through  the  blue-green  with  a trans- 
mission band  in  the  violet — that  is  to  say,  a brown  is  really  a 
degraded  orange,  and  is  a variant  on  the  colour  described  as 
orange,  resulting  from  a single  absorption  band  in  the  blue- 
green. 

It  i^  clear,  therefore,  that  if  a thing  is  coloured  sky-blue  it 
means  that  it  is  absorbing  the  deep  red,  a violet-blue  object 
absorbs  the  orange,  a purple  the  yellow-green,  a magenta  the 
central  green,  an  orange  the  blue,  a yellow  the  blue-violet,  and 
a lemon-yellow  only  the  extreme  violet.  If  a sky-blue  objedt  be 
looked  at  through  a piece  of  yellow  glass  it  will  be  found  to 
look  bright  green  in  colour,  so  that  a green  colour  is  produced 
by  the  absorption  both  of  the  red  and  of  the  blue,  the  blue 
objedt  absorbing  the  red  light  and  the  yellow  glass  the  blue 
light. 

Natural  colours  do  not  generally  show  sharp  absorption  bands, 
though  the  absorption  bands  produced  by  the  stains  used  in 
microscopy  are  mostly  fairly  sharp.  The  same  rule  holds  true, 
however;  if  a magenta  objedt  in  nature  does  not  signify  a clean 
sharp  absorption  band  in  the  green  it  still  means  that  that  objedt 
absorbs  far  more  of  the  green  than  of  any  other  colour,  and,  as 

14 


THE  NATURE  OF  COLOUR 


regards  photography,  we  can  apply  the  rules  deduced  from 
theoretical  residuals  to  natural  colours.  These  rules  are  as 
follows : 

i.  If  a colour  is  to  be  rendered  as  black  as  possible  then  it 
must  be  photographed  by  light  which  is  completely  absorbed  by 


Fig.  7.  Theoretical  and  Actual  Violet 


the  colour;  that  is,  by  light  of  the  wave-lengths  comprised 
within  its  absorption  band. 

2.  The  second  rule  deals  with  the  case  where  contrast  is 
required,  not  against  the  background  but  within  the  object 
itself. 

The  proper  procedure  in  this  case  is  to  photograph  the  object 
by  the  light  which  it  transmits. 

In  fig.  7 we  see  a sharp  absorption  band  depidted,  which 


4,000  5.000  6,000  7.000 

Fig.  8.  Theoretical  and  Actual  Green 


would  give  rise  to  a violet-blue  colour.  An  actual  violet-blue, 
however,  will  have  an  absorption  band  of  the  type  shown  by  the 
shading  in  the  opposite  direction  on  the  diagram. 

Similarly,  fig.  8 shows  an  ideal  green,  having  sharp  red  and 
violet  absorption  and  an  adtual  green  with  its  gradual  ending 
and  absorption  of  the  green  itself. 

The  sharpness  of  absorption  bands  is  of  great  importance  in 
rcspedt  of  the  luminosity  of  the  colours  produced  by  them. 

*5 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


The  side  of  an  absorption  band,  which  is  toward  the  red  end 
of  the  spedtrum,  generally  has  a sharp  edge,  as  shown  in  fig.  7, 
while  that  which  is  toward  the  blue  end  has  a gradual  edge,  a 
considerable  amount  of  absorption  remaining  even  in  the  trans- 
mitted portions  of  the  spedtrum.  As  a result,  colours  which  are 
bounded  by  the  sharp  edges — that  is,  reds,  oranges,  and  yellows — 
are  bright  colours,  while  colours  which  are  bounded  by  the 
gradual  edges — blue-greens,  blues,  and  violets — are  dark  colours. 
A green  will,  as  a general  rule,  have  a sharp  edge  at  its  blue 
limit  and  a gradual  edge  at  its  red  limit,  and  will  consequently 
be  of  intermediate  brightness. 

If  we  divide  the  spedtrum  at  5,000  and  at  6,000  so  that  we 
get  three  portions,  4,000  to  5,000  which  we  may  term  blue- 
violet,  5,000  to  6,000  which  we  may  term  green,  and  6,000  to 
7,000  which  we  may  term  red,  then  examination  of  the 
luminosity  curve,  given  in  Chapter  II,  fig.  10,  will  show  that 
about  of  the  whole  light  should  be  “ green,” about  ^should 
be  u red,”  and  about  should  be  “blue.”  But  inasmuch  as  a 
bright  red  objedt  will  refledt  nearly  all  the  incident  “ red  ” light, 
while  a bright  green  objedt  will  only  refledt  about  ^ of  the 
“green”  light,  and  a bright  blue  objedt  J of  the  “ blue”  light; 
a red  objedt  will  be  the  brightest,  a green  objedt  less  bright, 
and  a blue  objedt  very  dark  indeed. 

A yellow,  having  only  a single  sharp  absorption  edge,  is  very 
bright.  A yellow  objedt  usually  refledts  even  more  red  light 
than  a red  objedt,  and  much  more  green  light  than  a green 
objedt. 

Dr.  Mees  made  a number  of  measurements  of  the  absorption 
by  various  filters  and  colours  of  the  light  which  they  are  sup- 
posed to  transmit  or  refledt,  with  the  following  results: 


Pure  Dye  Filters 


Transmitting. 


Region  for  which  Trans- 

Transparency  was  parency, 

measured.  per  cent. 


5,900  to  red  end  (tricolour  red) 

5,900  » » 

4,800  to  6,000  (tricolour  green)  . 
4,800  to  6,000  „ 

4,000  to  5,100  (tricolour  blue) 
4,000  to  5,100  „ 

4,000  to  4,700  (D)  (methyl  violet) 

5.600  to  red  end  (E)  . 

5.600  „ 


Same  filter  75 

6,100  to  red  end  78 

Same  filter  32 

4.900  to  5,8oo  35-5 


Same  filter  1 1*5 

4,000  to  4,800  1 6-5 


4,000  to  4,600  15 

Same  filter  69 

5,900  to  red  end  85 


THE  NATURE  OF  COLOUR 


Region  for  which 

Trans- 

Transmitting. 

Transparency  was 

parency, 

measured. 

per  cent. 

5,100  to  red  end  (G) 

Same  filter 

79 

5,ioo  „ 

5,600  to  red  end 

89 

4,000  to  5,400  (H) 

. Same  filter 

14 

4,000  to  5,400  „ 

. 4,600  to  5,100 

16 

4,600  to  red  end  (K2)  . 

Same  filter 

72‘5 

4,600  „ 

• k3 

85 

K3 

• k3 

81 

K3 

. 5,ioo  to  red  end 

86 

Light  naphthol  green  (about  4,500 

to  6,500) .... 

. 4,900  to  5,700 

40 

Dark  naphthol  green  . 

. 4,900  to  5,700 

i4*5 

Xylen  red  (4,000  to  5,100),  purest 

blue  obtainable 

. 4,000  to  4,700 

41 

The  chief  points  of  interest  are  the  luminosity  of  the  yellows 
(K2,  K3,  G);  orange  (E);  and  red  (A);  the  darkening  in  the 
greens,  and  even  more  in  the  blues  and  violets. 


Printing  Inks 

Bright  scarlet 

5,900  to  red  end 

83*5 

V)  55 

. 6,100  „ 

88 

Bright  light  blue  . 

. 4,000  to  5,100 

42 

Dyed  Wools 


These  were  kindly  given  by  Dr.  E.  Koenig,  of  Hochst  a/M, 
who  stated  that  he  considered  them  to  be  very  good  pure 
colours. 


Colour. 

Region  of 

Percentagt 

Measurement. 

Reflected. 

Dark  purple 

. . 4,000  to  4,800 

9*5 

Bright  „ 

. 4,000  to  4,800 

22 

» » * 

. . 6,500  to  red  end 

42 

Dark  blue  . 

. . 4,000  to  4,700 

l9 

Light  „ . . . 

. 4,000  to  5,100 

18 

Dark  blue-green  . 

. 5,000  to  5,800 

1 1 

Light  „ 

. 4,900  to  5,400 

25 

Dark  yellow-green 

. 5,000  to  5,800 

17-5 

Bright  „ 

. 4,800  to  6,100 

. . 5,000  to  5,800 

26-5 

» » 

39 

Yellow 

. . 5,100  to  red  end 

67 

17 

B 

PHOTOGRAPHY  OF  COLOURED  OBJECTS 


Colour. 


Orange 

Scarlet 

Bright  scarlet 
Deep  red 


Region  of  Percentage 

Measurement.  Refle&ed. 


5,600 

57*5 

5,900 

70 

5,9°° 

61 

6,100 

68 

6,100 

71 

6,100 

49 

6,500 

77 

18 


CHAPTER  II 


THE  SENSITIVENESS  TO  COLOURED  LIGHT  OF  THE  EYE  AND 
OF  PHOTOGRAPHIC  PLATES 

WE  have  seen  that  the  eye  distinguishes  light  of  different 
wave  lengths  by  the  produdfion  of  an  appearance  of 
colour ; thus  a ray  of  light  containing  waves  of  a length  of  4,600 
of  our  units  would  be  called  violet,  while  if  the  waves  were  of 
the  length  of  6,500  the  resultant  impression  on  the  eye  would 
be  said  to  be  deep  red.  But  the  sensitiveness  of  the  eye  is  not 
the  same  for  waves  of  different  lengths,  and  if  we  attempt  to 
represent  in  monochrome  the  band  of  coloured  light  called  the 
spedlrum  as  it  appears  to  the  eve,  it  will  look  something  like 


^ A A 

COLOUR  Invisible  Limit  ot  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 

TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  9.  The  Luminosity  value  of  the  Spectrum  as  it  appears 

to  the  Eye 

fig.  9,  the  yellow-green  light  appearing  brightest  and  the  yellow, 
orange,  and  red  light  on  one  side,  green,  blue-green,  and  blue  on 
the  other  appearing  progressively  darker  until  the  violet  appears 
very  dark  and  the  visible  spe&rum  ends. 

The  eye  cannot  perceive  at  all  waves  below  4,000  units,  i.e.y 
what  is  known  as  ultra-violet  light;  neither  can  it  perceive  rays 
which  are  above  7,000  units,  so  that  to  these  we  must  regard 
the  eye  as  insensitive.  The  eye  is  very  little  sensitive  to  the 
extreme  violet  rays  between  4,000  and  4,500.  The  blue  affects 
it  more  and  appears,  as  we  say,  bright.  Between  5,000  and 
6,000  the  green  appears  as  the  brightest  part  of  the  spectrum  ; 
above  6,000  we  have  the  bright  reds,  but  the  intensity  rapidly 

" 19 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

falls  off  as  the  waves  get  longer,  until  beyond  7,000  we  see 
practically  nothing.  We  may  also  draw  a curve  showing  the 
sensitiveness  of  the  eye  to  the  spedlrum.  It  will  be  noted 
that  this  curve  has  a maximum  at  about  wave  length  5,900,  but 
this  only  holds  for  intense  light.  As  the  intensity  of  the  light 
diminishes,  not  merely  does  the  eye  see  less,  but  the  relative  sensi- 
tiveness of  the  colours  changes  somewhat,  shifting  towards  the 
blue.  This  is  what  is  known  as  “ Purkinje’s  Phenomenon.” 
The  explanation  offered  for  it  by  Professor  Schaum  is  suffici- 
ently interesting  and  little  known  to  be  worth  repetition.  It  is 
known  that  the  retina  consists  of  rods  and  cones,  of  which  the 
cones  are  considered  to  be  colour-sensitive,  and  the  rods  colour- 
blind. In  the  part  of  the  retina  exactly  opposite  the  centre  of 
the  pupil  there  is  a small  depression  which  contains  no  rods,  but 
only  cones,  and  here  it  is  found  that  the  Purkinje  phenomenon 


4.000  5,000  sooc*  7000 

Fig.  10. 


is  non-existent,  so  that  the  intensity  maximum  remains  con- 
stant. So  that  we  may  conclude  that  the  colour-sensitive  cones 
alone  display  no  Purkinje  phenomenon,  and  that  the  phenomenon 
is  due  to  the  association  of  these  cones  with  the  colour-blind 
rods.  It  is  found  that  the  sensitiveness  curve  for  this  region 
containing  only  cones  is  identical  with  the  curve  of  sensitiveness 
for  great  intensities  of  light,  so  that  this  is  the  curve  of  the 
cones.  On  the  other  hand,  since  the  rods  are  much  more  sensi- 
tive to  feeble  intensities  of  light  than  the  cones,  as  is  shown  by 
the  fa£t  that  the  sense  of  light  remains  after  colour  can  no  longer 
be  distinguished,  the  sensitiveness  curve  of  the  rods  will  corre- 
spond with  thecurve  for  minimum  intensity ; so  that  for  minimum 
intensity  the  sensitiveness  curve  is  due  to  the  rods  alone,  and  as 
the  intensity  grows,  the  curve  is  more  and  more  influenced  by 
the  cones,  until  with  maximum  intensity  the  curve  of  sensitive- 
ness is  almost  entirely  determined  by  the  cones. 

It  is  for  the  reason  that  in  very  weak  lights  the  eye  has  a 

20 


SENSITIVENESS  OF  THE  EYE  AND  PLATE 


maximum  sensitiveness  to  a particular  colour,  namely,  a green, 
that  the  safelights  supplied  for  use  in  the  dark-room  when 
Wratten  panchromatic  plates  are  handled  are  of  this  green 
colour.  The  plate  is  sensitive  to  all  colours,  but  an  amount  of 
green  light  can  be  used,  if  discretion  is  shown,  that  is  sufficient 
to  see  by  without  too  much  danger  of  fogging  the  plate,  whereas 
if  a red  were  to  be  chosen,  so  much  more  would  have  to  be  used 
for  it  to  make  objedts  visible  that  the  plate  would  inevitably  be 
fogged. 

Just  as  the  eye  is  unequally  sensitive  to  light  of  different 
colours,  so  a photographic  plate  is  unequally  sensitive  to  light 
of  different  colours.  If  we  take  an  ordinary  photographic  plate 
and  measure  its  sensitiveness,  we  shall  find  that  it  differs  very 
markedly  from  that  of  the  eye.  The  eye  can  see  waves  of  no 
shorter  length  than  4,000  units;  a photographic  plate  can  see 
very  much  shorter  waves,  and  can  detedt  light  which  is  quite 
invisible  to  the  eye,  this  light  being  usually  called  ultra-violet, 
because  it  is  beyond  the  violet.  Also  the  maximum  of  sensitive- 
ness of  an  ordinary  plate  is  in  the  violet,  and  all  the  red,  orange, 
and  nearly  all  the  green  light  is  invisible  to  it.  That  is  to  say, 
the  ordinary  plate  perceives  objedts  only  by  the  blue  and  violet 
light  which  they  refledl,  and  this  is  a grave  fault  in  the  plate 
when  regarded  as  an  instrument  for  perceiving  and  recording 
coloured  objedts,  because  the  record  which  it  makes  of  coloured 
objedts  differs  entirely  from  that  which  the  eye  makes. 

It  was  found  by  Vogel  in  1873  that,  by  treating  plates  with 
dyes,  they  could  be  given,  besides  their  usual  sensitiveness,  a 
secondary  sensitiveness  in  approximately  the  region  of  the 
spectrum  which  those  dyes  absorb.  Thus  if  a plate  is  treated 
with  a solution  of  erythrosine  which  absorbs  the  yellowish 
green,  it  will  be  sensitive  to  the  yellow-green,  besides  being 
sensitive  to  the  blue  and  violet.  Plates  which  have  been  treated 
in  this  way  are  those  which  are  known  as  u orthochromatic,” 
the  word  implying  that  they  can  render  objedts  in  their  true 
colour  values.  The  ordinary  commercial  orthochromatic  plate, 
which  is  usually  made  by  putting  some  eosine  or  erythrosine 
into  the  emulsion,  has  a sensitiveness  curve  of  the  type  shown 
in  fig.  1 1 (b),  and  it  will  be  seen  at  once,  on  comparing  this  with 
the  sensitiveness  curve  of  the  eye,  that,  although  the  plate  is 
certainly  better  in  consequence  of  this  treatment  with  erythro- 
sine, it  cannot  be  described  as  at  all  comparable  in  sensitiveness 
with  the  eye.  It  has  an  enormous  excess  of  sensitiveness  in  the 
blue  and  violet,  it  has  the  sensitiveness  to  the  ultra-violet  which 
the  eye  has  not  at  all,  it  then  has  very  little  sensitiveness 

21 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

indeed  to  the  blue-green,  a small  maximum  of  sensitiveness  in 
the  yellow-green,  and  an  absence  of  sensitiveness  to  the  red.  It 
may  be  assumed  that  if  we  take  the  blue  to  include  the  whole 
spedrum  up  to  5,000,  the  green  to  be  the  spe&rum  from  5,000 
to  6,000,  and  the  red  from  6,000  upwards,  that  the  sensitive- 
ness of  the  ordinary  orthochromatic  plate  is  distributed  in  the 


3,000  3,500  4,000  4,500  5,000  5,500  6,000  6,500  7,000 


(B)  Ordinary  Ortho  or  Isochromatic  Plate 


3,000  3,500  4,000  4,500  5,000  5,500  6,000  6,500  7,000 


Fig.  11.  Different  Types  of  Plate  Sensitiveness  Curves 
to  Daylight 

ratio  of  40  parts  in  the  blue,  one  part  in  the  green,  and  none  in 
the  red.  If  we  assume  for  the  sake  of  argument  that  the  eye 
sees  the  three  parts  of  the  spectrum  as  of  equal  intensity,  then 
the  orthochromatic  plate,  besides  the  fa<5t  that  it  is  not  sensitive 
to  the  red,  has  only  -fo  of  the  sensitiveness  in  the  green  that  it 
would  require  to  be  equal  in  sensitiveness  to  the  eye. 

If,  however,  instead  of  sensitising  a plate  in  the  wav  we  have 
described,  we  bathe  the  finished  plate  in  a solution  of  certain  of 

22 


SENSITIVENESS  OF  THE  EYE  AND  PLATE 


the  dyes  called  isocyanines,  we  can  prepare  a plate  which  is  very 
much  more  sensitive  both  to  the  green  and  to  the  red.  Messrs. 
Wratten  and  Wainwright,  Ltd.,  were  the  first  to  succeed  in 
preparing  a plate  commercially  in  this  manner,  the  plate  being 
sensitive  to  both  green  and  red,  and  this  plate  they  called  the 
“Wratten  Panchromatic  Plate.”  The  plate  is  sensitive  to  the 
whole  visible  spe&rum  ; although  it  has  a considerable  excess 
of  sensitiveness  in  the  blue,  this  excess  is  very  much  less  than  in 
the  case  of  the  ordinary  orthochromatic  plates,  and  there  are  no 
absences  of  sensitiveness  throughout  the  whole  spectrum.  The 
distribution  of  sensitiveness  in  this  plate  is  also  shown  in  fig.  io, 
and  it  may  be  said  that  of  its  sensitiveness  is  in  the  blue,  in 
the  green,  and  ^ in  the  red ; so  that  the  sensitiveness  to  blue 
is  seven  times  too  great  compared  with  the  rest  of  the  spe&rum, 
while  the  sensitiveness  to  green  and  red  together  is  §-  of  that 
required  to  have  the  same  sensitiveness  as  the  eye. 

In  order  to  attain  the  same  relative  sensitiveness  as  the  eye, 
it  is  necessary  with  an  ordinary  orthochromatic  plate  or  with 
the  panchromatic  plate,  to  use  absorbing  colour  filters  which 
shall  diminish  the  excess  of  blue  light,  and  it  is  the  considera- 
tion of  these  colour  filters  and  of  the  effect  which  they  will  have 
on  the  total  sensitiveness  of  the  plate,  which  must  now  be  con- 
sidered. 


23 


CHAPTER  III 


ORTHOCHROMATIC  FILTERS 

THE  need  for  orthochromatic  filters  in  photography  is  still 
insufficiently  realized  by  many  workers.  For  so  many  years 
photographers  have  been  accustomed  to  the  incorrect  rendering 
of  coloured  objedts  in  monochrome  which  is  given  by  ordinary 
photographic  plates  that  a kind  of  photographic  convention  has 
been  set  up  in  their  minds,  so  that  a picture  of  a landscape  in 
which  grass  is  rendered  as  a dark  patch,  and  a blue  sky  is 
almost  white  paper,  is  accepted  without  any  feeling  of  its  in- 
correctness; the  more  experienced  a photographer,  indeed,  the 
more  fixed  this  convention  becomes,  so  that  photographs  taken 
under  conditions  which  corredtly  reproduce  the  luminosities  of 
the  subjedt  may  sometimes  appear  over-corredted  to  a worker 
who  has  become  accustomed  to  a “ photographic  rendering,” 
and  in  whose  mind  the  reproduction  of  a scarlet  as  dead  black 
would  raise  no  question  whatever. 

Recently,  however,  many  photographers  have  become  more 
critical  in  this  respeCt,  and  it  is  beginning  to  be  recognized  that, 
so  far  as  possible,  photographs  should  corredtly  translate  into 
monochrome  the  luminosity  values  of  colour  as  seen  by  the  eye, 
and  for  this  purpose  corredtly  adjusted  orthochromatic  filters 
and  plates  are  a necessity. 

In  order  that  we  may  understand  the  need  for  an  ortho- 
chromatic filter  let  us  take  a simple  example  of  a coloured 
objedt  such  as  that  presented  by  a yellow  daffodil.  If  we  photo- 
graph side  by  side  a daffodil  and  a narcissus  on  an  ordinary 
photographic  plate  we  shall  find  that  although  to  the  eye  the 
yellow  daffodil  appears  almost  as  bright  as  the  white  narcissus, 
yet  in  the  print  (fig.  12)  the  daffodil  appears  much  darker,  the 
difference  being  especially  marked  in  the  more  deeply  coloured 
trumpet  of  the  flower.  Clearly  the  light  which  is  refledted  from 
the  daffodil  is  deficient  in  some  essential  constituent  which  has 
much  adtion  on  the  photographic  plate,  although  its  loss  does 
not  make  the  flower  seem  much  darker  to  the  eye.  If  we 
examine  the  light  refledted  from  the  flower  by  means  of  a 

24 


ORTHOCHROMATIC  FILTERS 


spectroscope  it  will  at  first  sight  appear  that  we  have  the  same 
spectrum  as  we  got  at  first  from  white  light,  but  on  closer  in- 
spection we  shall  see  that  while  the  green,  orange,  and  red 
regions  are  fully  present,  the  blue  light  is  dimmed  and  the 
violet  light  is  almost  completely  absent.  We  thus  see  that  the 
reason  why  the  daffodil  looks  different  from  a white  flower, 
appears  “yellow”  in  faCt,  is  that  it  fails  to  refleCt  all  the  con- 
stituents of  white  light,  and  absorbs  the  violet  and  blue  con- 


Fig.  i2.  Daffodil  and  Narcissus  on  Ordinary  Plate 

stituents.  These  violet  and  blue  constituents  of  white  light  are 
thus  shown  to  be  those  which  have  a strong  aCIion  upon  a photo- 
graphic plate,  although  to  the  eye  they  appear  dark. 

The  frontispiece  of  this  book  shows  a chart  which  is  intended 
to  represent  the  spe&rum  colour  both  in  hue  and  also  approxim- 
ately in  brightness  and  the  strong  aCfion  of  the  violet  and  blue 
rays,  and  the  feeble  aCtion  of  the  green,  orange,  and  red  rays  is 
shown  in  fig.  13,  which  is  a photograph  of  the  frontispiece 
taken  upon  an  ordinary  fast  plate.  In  this  photograph  the 

25 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

violet,  blue,  and  blue-green  patches,  which  are  darkest  to  the 
eye,  are  reproduced  light,  while  the  rest  of  the  chart  appears 
dark. 

If  we  take  a photograph  of  the  spe&rum  of  white  light  upon 


Fig.  13.  Colour  Chart  on  Ordinary  Plate 


this  plate  we  get  the  result  shown  in  fig.  14.  It  will  be  seen  that 
the  plate  sees  the  coloured  rays  in  a very  different  manner  from 
the  eye.  The  red,  orange,  and  green  colours  which  are  bright 


COLOUR  Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 

TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  14.  The  Luminosity  Value  of  the  Spectrum  as  it  appears 
to  an  Ordinary  Plate 


to  the  eye  appear  quite  dark  to  the  plate,  while  the  deep  violet 
light  which  is  very  dark  to  the  eye  is  the  brightest  colour  to 
the  plate,  and  in  addition  the  plate  will  demonstrate  that 
there  are  rays  beyond  the  violet  which  are  quite  invisible  to 
the  eye. 


26 


ORTHOCHROMATIC  FILTERS 


COLOUR 
TO  EYE 


These  ultra-violet  rays  are  very  strong  in  daylight  and  play 
an  important  part  in  the  economy  of  the  universe,  being  the 


Fig.  15.  Daffodil  and  Narcissus  on  Panchromatic  Plate 
(unscreened) 

chief  cause  of  most  of  the  effects  which  are  produced  by  sun- 
light, such  as  the  tanning  of  the  skin,  the  fading  of  coloured 


A 

Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 
Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  16.  The  Spectrum  photographed  on  a Panchromatic 

Plate 

cloths,  or  the  development  of  plants.  They  comprise,  indeed, 
those  constituents  of  white  light  which  produces  the  “ chemical  ” 

" 27 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

effedts  of  light,  and  naturally  they  play  a great  part  in  the 
essentially  chemical  adtion  of  the  exposure  of  a photographic 
plate. 

The  insensitiveness  of  a plate  to  the  colours  which  are  bright 
to  the  eye,  and  sensitiveness  to  those  which  are  dark  to  the  eye 
are  of  much  importance  in  photography;  in  landscape  photo- 
graphs, for  instance,  the  grass,  which  because  it  absorbs  the 
violet  and  blue  rays  and  also  some  of  the  red  rays,  appears 


Fig.  17.  Taken  on  a Wratten  Panchromatic  Plate  with 
K3  Filter 

green,  is  always  reproduced  too  dark,  and  white  clouds  are  lost 
against  the  blue  sky,  although  to  the  eye  they  appear  much 
brighter,  because  the  light  from  the  blue  sky  is  deficient  in  red 
rays,  and  these  rays  being  bright  to  the  eye  their  absence  pro- 
duces a strong  efFedt.  To  the  plate,  however,  which  is  blind  to 
the  red  rays,  their  presence  or  absence  is  indifferent,  and  con- 
sequently the  blue  skv  and  white  clouds  appear  of  nearly  the 
same  intensity. 

If  we  photograph  the  daffodil  and  narcissus  on  a Wratten 
panchromatic  plate  we  shall  obtain  a result  which  approximates 

28 


ORTHOCHROMATIC  FILTERS 


much  more  closely  to  the  truth  than  did  that  which  we  got 
with  the  ordinary  plate,  fig.  12.  It  will  be  noticed  that  in 
order  to  photograph  the  flowers  thev  were  placed  in  a vase;  this 
was  a white  vase  with  a blue  landscape  painted  upon  it,  and 
although  the  daffodils  appear  bright  in  the  photograph,  as  they 
do  to  the  eye,  yet  the  blue  design  on  the  vase  is  almost  invisible, 
although  to  the  eye  it  stands  out  most  distindly. 

A comparison  of  fig.  16  with  fig.  9 will  show  the  reason  for 
this;  the  panchromatic  plate  is  sensitive  to  green,  orange,  and 
red  light,  just  as  the  eye  is,  but  it  is  still  very  much  too  sensitive 


Fig.  18.  Colour  Chart  on  Wratten  Panchromatic  Plate 
with  K3  Filter 

to  the  blue  and  violet  light,  and  to  the  invisible  ultra-violet  rays. 
Owing  to  the  great  intensity  of  these  blue,  violet,  and  ultra- 
violet rays  in  daylight,  most  of  the  photographic  action,  even  on 
a panchromatic  plate,  is  produced  by  them,  so  that  the  advant- 
age  gained  by  sensitising  the  plate  to  the  green  and  red  rays 
largely  is  lost  by  their  effed  being  drowned  by  the  violet  and 
ultra-violet  rays.  With  artificial  light  (excepting  mercury 
vapour  and  enclosed  arc)  the  more  adinic  rays  are  weak,  and  a 
panchromatic  plate  gives  at  once  manifestly  better  results  than 
an  ordinary  plate. 

In  daylight  we  can  secure  a similar  result  if  we  modify  the 
light  reaching  the  plate  by  passing  it  through  a filter  or  screen 

29" 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

(usually  attached  to  the  lens),  which  removes  all  the  ultra-violet 
light  and  as  much  of  the  blue  and  violet  light  as  is  necessary. 
Such  a filter  or  screen  is  called  an  a orthochromatic  ” or  M isochro- 
matic”  filter. 

Fig.  i 7 shows  a photograph  of  the  daffodils  and  narcissi  in  the 
vase  taken  through  such  a filter  on  a W ratten  panchromatic 
plate,  and  it  will  be  seen  that  not  only  are  the  flowers  rendered 
corredtly  in  their  relative  tone  values,  but  also  the  design  on  the 
vase  is  clearly  defined,  as  it  appears  to  the  eye. 

In  the  same  way  fig.  18  shows  a photograph  of  the  frontis- 
piece taken  through  a corre&ly  adjusted  (K3)  filter  on  a 
panchromatic  plate  and  represents  the  colours  of  the  chart  in 
their  relative  luminosities  as  they  appear  to  the  eye. 


30 


CHAPTER  IV 


THE  EFFICIENCY  OF  ORTHOCHROMATIC  FILTERS 

WE  have  seen  that  orthochromatic  filters  are  designed  to 
remove  the  ultra-violet  and  so  much  of  the  violet  light  as 
is  necessary  to  compensate  for  the  extra  sensitiveness  of  the 
plate  to  those  rays. 

Now  in* removing  this  light  the  orthochromatic  filter  increases 
the  necessary  exposure,  because  if  we  remove  those  rays  to 
which  the  plate  is  most  sensitive  we  must  compensate  for  it  by 
exposing  the  plate  fora  longertime  to  the  adfion  of  the  remain- 
ing rays,  and  the  amount  of  this  increased  exposure  will  clearly 
be  dependent  both  on  the  proportion  of  the  violet  and  the  blue 
rays  which  are  removed  by  the  orthochromatic  filter,  and  also 
upon  the  sensitiveness  of  the  plate  for  the  remaining  rays  (green, 
orange,  and  red),  which  are  not  removed  by  the  filter. 

The  number  of  times  by  which  the  exposure  must  be  in- 
creased for  a given  filter  with  a given  plate  is  called  the  multi- 
plying fattor  of  the  filter,  and  this  depends  upon  the  plate  with 
which  it  is  used.  It  is  meaningless  to  refer  to  filters  as  “ two 
times  ” or  “ four  times  ” filters. 

Now,  since  it  is  always  desirable  that  we  should  be  able  to 
give  as  short  an  exposure  as  possible,  what  is  required  in  a filter 
is  that  it  should  produce  the  greatest  possible  effedt  with  the 
least  possible  increase  of  exposure,  so  that  a filter  will  be  con- 
sidered most  efficient  when  it  produces  the  maximum  result 
with  the  minimum  multiplying  factor. 

The  ideal  filter  will,  therefore,  absorb  all  the  ultra-violet 
light,  and  as  much  as  is  needful  of  the  violet  and  blue  light,  but 
will  transmit  all  the  orange  and  red  light  which  falls  upon  it. 
If  a filter  transmits  any  of  the  ultra-violet  light,  which  it  should 
absorb,  or  absorbs  any  of  the  green,  orange,  or  red  light,  which 
it  should  transmit,  then  it  will  be  more  or  less  inefficient 
from  that  cause.  An  inefficient  filter  will,  therefore,  have  a 
high  multiplying  factor  compared  with  the  correction  which  it 
will  give,  while,  on  the  other  hand,  a low  multiplying  factor  for 

31 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

a filter  may  be  due  simply  to  insufficient  correction.  An  efficient 
filter  will  have  a low  multiplying  factor  but  will  also  give  good 
correction. 

Some  of  the  earlier  filters  were  made  of  yellowish  brown 
glass,  and  a few  such  filters  are  still  issued.  Such  filters  are  very 
inefficient,  producing  only  a small  degree  of  correction  because 
they  transmit  a good  deal  of  the  ultra-violet  and  violet  light, 
while  at  the  same  time  they  require  a very  considerable  increase 
of  exposure  because  they  absorb  much  of  the  green  light  and 
even  some  of  the  orange  and  red  rays,  which  should  be  com- 
pletely transmitted.  Much  the  same  objections  apply  to  the 


Fig.  19.  Diagram  painted  in  Chinese  White  on  White  Card 

filters  which  are  sometimes  met  with  made  ot  green  glass. 
These  usually  absorb  the  ultra-violet  and  violet  light  in  a fairly 
satisfactory  way,  but  the  very  strong  absorption  which  they 
possess  for  red  light  causes  an  unnecessarily  great  increase  in 
exposure  if  they  are  used  with  panchromatic  plates,  while,  as 
they  absorb  a considerable  amount  of  the  green  light  they  can- 
not be  considered  efficient  even  if  ordinary  orthochromatic 
plates,  insensitive  to  the  red,  be  used. 

For  orthochromatic  work  all  filters  which  are  not  a clear 
yellow  should  be  disregarded ; if  the  filter  is  laid  down  on  a 
sheet  of  white  paper  it  should  appear  a bright  or  pale  yellow, 
according  to  its  depth,  but  it  should  not  appear  brown,  and  if  it 

32 


EFFICIENCY  OF  ORTHOCHROMATIC  FILTERS 


does  appear  brown,  then  it  will  be  unsatisfactory  in  correction, 
and  will  require  an  unnecessarily  long  exposure.  Compared 
with  a modern  u K ” filter,  a brown  glass  filter  is  as  inefficient 
for  photographic  purposes  as  a speCtacle  lens  compared  with  a 
modern  anastigmat. 

Nor  is  it  sufficient  for  an  orthochromatic  filter  to  be  yellow, 
for  a yellow  filter  may  still  be  inefficient,  and  the  great  criterion 
as  to  the  efficiency  of  an  orthochromatic  filter  which  is  clear 
yellow  in  colour,  and  which  therefore  absorbs  only  a minimum 
amount  of  the  rays  which  it  should  transmit,  is  that  the  filter 
should  completely  absorb  the  ultra-violet  light. 

We  have  seen  that  a photographic  plate  is  extremely  sensitive 
to  ultra-violet  light.  Now  some  substances  which  are  quite 
without  colour  to  the  eye  strongly  absorb  ultra-violet  light ; 
Chinese  white,  for  instance,  which  is  often  used  by  artists  for 
the  high  lights  in  drawings,  and  which  appears  quite  white  to 
the  eye,  absorbs  ultra-violet  light,  so  that  when  the  drawings 
are  photographed  by  an  arc  lamp  upon  wet  collodion  plates, 
which  are  chiefly  sensitive  to  the  ultra-violet,  the  Chinese  white 
appears  a dirty  grey.  Fig.  19  shows  such  a photograph  of  a 
diagram  painted  in  Chinese  white  on  a white  card,  the  diagram 
being  almost  invisible  to  the  eye  but  photographing  as  shown. 

Moreover,  ultra-violet  light  is  far  more  easily  scattered  by 
traces  of  mist  in  the  atmosphere  than  visible  light  is,  so  much 
so,  that  when  Professor  R.  W.  Wood  took  photographs  by 
means  of  the  ultra-violet  light  only,  using  a special  apparatus, 
he  found  that  if  one  could  see  the  rays  which  he  was  using,  even 
the  clear  atmosphere  of  the  United  States  would  appear  to  be 
continually  filled  with  mist;  so  that  the  well-known  photo- 
graphic haze  which  so  often  spoils  the  distance  in  photographs 
taken  on  ordinary  plates  is  due  to  the  ultra-violet  light,  and  our 
orthochromatic  filter  must  be  adjusted  to  cut  out  all  of  the 
ultra-violet  light  and  just  so  much  of  the  violet  light  as  is  neces- 
sary to  produce  exactly  the  effect  of  “ atmosphere  ” which  is 
seen  by  the  eye.  If  too  much  violet  light  is  removed  by  the 
filter  all  effect  of  atmosphere  will  be  lost ; but  this  efFedf,  known 
as  “over-correction,”  will  be  discussed  later. 

Now  some  yellow  dyes,  while  removing  violet  light  quite 
satisfactorily,  transmit  a great  deal  of  ultra-violet  light,  and  it  is 
indeed  possible  to  use  one  such  dye  to  produce  an  anti-ortho- 
chromatic  filter;  that  is,  a filter  which  will  exaggerate  instead 
of  diminishing  the  false  tone  rendering  to  which  photographic 
plates  are  prone. 

Until  a few  years  ago  the  dyes  which  were  mostly  used  for 

33  c 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

the  making  of  orthochromatic  filters,  while  they  gave  clear 
yellow  films  and  were  stable  to  light,  were  unsatisfa&ory  in 
that,  except  when  very  strong,  they  transmitted  more  or  less 
ultra-violet  light,  and  only  the  introdudlion  of  new  dyes  a few 
years  ago  made  it  possible  for  the  first  time  to  prepare  ortho- 
chromatic filters  of  almost  ideal  efficiency,  combined  with  great 
stability  to  light.  Such  filters  are  prepared  by  us  under  the 
registered  name  of“K”  filters. 

To  illustrate  the  advance  which  the  introduction  of  these 
filters  marked  there  is  shown  in  figs.  20  and  21  two  photographs 
of  the  spectrum  produced  by  the  light  of  an  electric  arc  burning 
between  iron  poles,  the  first  taken  through  the  K2  filter,  the 
second  through  one  of  the  best  of  the  earlier  filters,  which  is  of 
almost  the  same  depth  to  the  eye  and  which  requires  about  the 
same  increase  of  exposure.  It  will  be  seen  that  while  the  green 


INVISIBLE  ULTRA-VIOLET  [VIOLET  BLUE.  GREEN  RED  . 

LIMIT  OF 
VISI6ILITV 


Fig.  20.  (a)  K Filter  Fig.  21.  (b)  Old  Filter 

and  red  portions  of  the  speCIrum  are  as  bright  through  the  K2 
filter  as  through  the  older  one,  the  violet  portion  is  much 
fainter  and  the  ultra-violet  is  altogether  absent,  while  the  old 
filter  is  shown  to  transmit  a very  considerable  amount  of  ultra- 
violet light. 

Since  the  use  of  a filter  is  to  compensate  for  the  excess 
sensitiveness  of  even  an  orthochromatic  or  panchromatic  plate 
to  the  violet  and  ultra-violet  rays,  it  follows  that  plates  of  differ- 
ent degrees  of  sensitiveness  will  require  filters  of  different  kinds 
to  produce  the  same  effect  as  is  seen  by  the  eye.  In  the  first 
place  it  is  clear  that  no  matter  what  filter  be  used,  an  ortho- 
chromatic plate  which  is  not  sensitive  to  red  can  never  render 
tone  values  as  they  appear,  from  the  very  fadt  that  the  plate  is 
red  blind  and  that,  except  in  a few  unfortunate  cases,  the  eye 
is  not.  The  most  perfect  filter,  in  fa£t,  with  such  a plate  can 
only  give  a result  similar  to  that  seen  by  a “colour-blind  ” per- 
son. But  even  so  it  is  not  a matter  of  indifference  what  filter  is 

34 


EFFICIENCY  OF  ORTHOCHROMATIC  FILTERS 


used  with  a non-red-sensitive  orthochromatic  plate.  If  the 
filter  be  too  strong  the  photograph  will  appear  over-corrected. 
This  over-corredtion  will  show  itself  chiefly  in  the  manner 
previously  referred  to,  that  is,  the  atmosphere  in  the  distance 
will  be  lost ; but  also  other  unpleasant  effects  may  be  observed. 
In  landscape,  the  sky  may  appear  too  dark  (this  is  often  also  the 
effedt  of  under-exposure)  and  light  grass  may  appear  almost 


COLOUR  Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 
TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  22.  Absorption  of  Sharp  cut  Filter 

white;  while  in  flower  photography,  yellow  flowers  maybe  in- 
distinguishable from  white  ones.  These  defects  are  produced 
by  a filter  which  too  completely  removes  the  violet  and  blue 
light,  as  depicted  in  the  diagram  (fig.  22),  instead  of  simply 
diminishing  them  to  the  required  extent,  as  shown  in  diagram 
(%.  23). 

Provided,  however,  that  a filter  is  satisfactory  in  this  respedt  and 


A A 

COLOUR  Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 

TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  23.  Absorption  of  Correct  Filter 


does  not  produce  over-correction,  while  at  the  same  time  it  com- 
pletely removes  ultra-violet  light,  little  is  gained  by  adjusting 
a filter  to  a special  orthochromatic  plate,  and  the  Ki,  Ki^,  and 
K2  filters,  which  have  almost  ideal  efficiency  and  are  free  from 
any  tendency  to  produce  over-correction,  will  give  results  as 
satisfactory  as  can  possibly  be  obtained  on  plates  which  are  not 
sensitive  to  red.  The  K3  filter,  however,  is  of  rather  a different 
type,  and  brings  us  to  the  consideration  of  panchromatic  plates. 

35 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

When  panchromatic  plates  are  used,  since  they  are  sensitive  to 
light  of  all  colours,  it  is  possible  to  use  a filter  which  will  produce 
upon  the  plates  a tone-rendering  identical  with  that  perceived 
by  the  normal  eye,  but  in  order  to  do  this  the  filter  must  be 
carefully  adjusted  to  the  plate  for  which  it  is  intended. 

The  older  plates  issued  as  panchromatic  had  very  little  sensi- 
tiveness to  red  and  it  was  consequently  necessary,  in  order  to 
get  correct  rendering,  to  use  with  those  plates  filters  which 
absorbed  not  only  ultra-violet,  violet,  and  blue  light,  but  also 
green  and  even  yellow-green  light,  thus  allowing  the  undimin- 
ished red  rays  to  produce  their  just  share  of  aCtion  upon  the 
plate.  Similarly  it  is  conceivable  that  a plate  having  an  excess 
of  sensitiveness  to  red  and  but  little  sensitiveness  to  green,  might 
require  a green  filter  to  absorb  the  red  rays  as  well  as  the  violet 
and  ultra-violet  rays  in  order  to  allow  the  green  to  aCt  more 


COLOUR  Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 

TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  24.  Photograph  of  the  Spectrum  on  an 
Orthochromatic  Plate 

fully.  All  such  filters  as  these  require  a very  great  increase  of 
exposure,  and  for  this  reason  they  came  but  little  into  use. 
But  fortunately  the  W ratten  panchromatic  plate  is  sensitive  in 
the  right  proportion  to  green,  orange,  and  red  light,  and  all  that 
is  necessary  to  produce  with  this  plate  an  exactly  correct  render- 
ing of  tone  values  is  a clear  yellow  filter,  removing  the  ultra- 
violet, and  absorbing  the  violet  and  blue  to  a slightly  greater 
extent  than  the  K2  filter,  thus  requiring  an  exposure  of  not 
more  than  five  times  that  needed  for  the  unscreened  plates. 
Such  a filter  is  the  K3  filter,  which,  however,  is  not  to  be  re- 
commended for  use  with  other  plates  than  the  panchromatic 
plates,  because,  as  a reference  to  fig.  24  will  show,  ordinary 
orthochromatic  plates  have  a band  of  insensitiveness  to  the  blue 
and  blue-green  rays,  and  the  K3  filter,  by  absorbing  these  rays 
to  some  extent,  accentuates  this  defeat. 

While  the  K3  filter  is  required  to  produce  full  correction  upon 
the  Wratten  panchromatic  plate,  so  that  the  photographic  ren- 

36 


EFFICIENCY  OF  ORTHOCHROMATIC  FILTERS 


dering  of  tone  values  is  the  same  as  that  perceived  by  the  eye, 
yet  very  satisfactory  results  are  obtainable  by  the  use  of  lighter 
filters.  From  the  point  of  view  of  correction  it  is  of  as  great 
importance  that  the  plate  should  be  strongly  sensitive  to  the 
green,  orange,  and  red  light  as  that  the  filter  should  be  efficient 
and  of  sufficient  depth,  so  that  a W ratten  panchromatic  plate 
used  without  a filter  at  all  will,  in  some  cases,  give  results  superior 
to  the  much  less  colour-sensitive  orthochromatic  plate  used  with 
a filter  increasing  the  exposure  four  or  five  times.  Moreover, 
the  correcting  aCtion  of  such  weak  filters  increases  with  the 
colour-sensitiveness  of  the  plate,  while  the  more  colour-sensitive 
the  plate  the  lower  the  multiplying  faCtor  of  the  filter.  Conse- 
quently, for  satisfactory  orthochromatic  work  the  first  essential 
is  that  the  plate  shall  be  as  colour-sensitive  as  possible,  and  then 
the  choice  of  filter  must  be  governed  largely  by  the  exposure 
which  can  be  given,  the  K3  being  used  where  full  correction 
is  desired,  and  where  the  duration  of  the  exposure  is  of  but 
little  importance.  For  all-round  work,  however,  the  K2  filter 
will  be  found  the  most  useful.  When  used  with  the  Wratten 
panchromatic  plate  it  gives  a degree  of  correction  slightly  less 
than  that  obtained  with  the  K3,  but  it  requires  only  two-thirds 
of  the  exposure  needed  for  the  deeper  screen. 

Where  short  exposure  is  of  greater  importance  than  full  cor- 
rection the  K.i  J or  the  Ki  filter  should  be  employed ; the  latter 
requires  only  half  the  exposure  needed  for  the  K2  and  notably 
improves  the  colour  rendering  as  compared  with  that  given  by 
the  unscreened  plate.  This  filter  is  also  largely  used  for  hand 
camera  work,  and  the  advantage  obtained,  even  with  such  a 
weak  filter,  is  very  manifest  in  the  results. 

In  some  classes  of  landscape  work  it  is  desirable  to  produce 
over-correCtion ; in  surveying  or  tele-photographic  work,  for 
instance,  where  the  utmost  clearness  and  detail  are  desired  rather 
than  a piCtorial  rendering,  it  is  necessary  to  remove  all  haze  and 
atmosphere,  and  for  this  purpose,  a strong  yellow  filter,  such  as 
the  Wratten  “ G ” filter,  is  best. 

Owing  to  the  depth  of  this  filter,  however,  it  can  only  satis- 
factorily be  used  with  panchromatic  plates,  because  with  less 
sensitive  plates  its  multiplying  faCtor  is  very  great,  and  the 
exposure,  which  in  any  case  is  often  considerable  in  tele-photo- 
graphy, becomes  impracticable. 

When  selecting  filters  for  telephoto  work  a K2  filter  should 
be  obtained  as  well  as  a “ G,”  because  in  many  cases  the  lighter 
filter  is  all  that  is  required,  and  it  has  the  advantage  of  requiring 
a shorter  exposure. 


37 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

^For  the  photography  of  cloud  forms  against  a blue  sky  a red 
filter  may  be  used  with  great  advantage,  and  the  u A”  filter  is 
very  suitable  for  this  work.  The  results  obtained  show,  of 
course,  a greatly  exaggerated  contrast,  but  if  the  form  of  the 
clouds  is  all  that  is  required  such  an  exaggeration  is  not  a dis- 
advantage, though  we  should  not  recommend  the  use  of  so  deep 
a filter  in  pidtorial  work. 


3* 


CHAPTER  V 


THE  MULTIPLYING  FACTOR  OF  ANY  SHARP-CUT  FILTER 

SUPPOSE  that  we  have  a filter  which  has  a perfedtly  sharp 
absorption — that  is  to  say,  which  cuts  a clean  sedtion  out 
of  the  spedtrum,  passing  only  light  between  two  definite  wave 
lengths,  and  without  any  absorption  of  that  light — then,  if  we 
wish  to  find  the  multiplying  fadtor  of  this  filter,  we  must  con- 
sider it  in  relation  to  the  sensitiveness  curve  of  the  plate. 

It  will  be  convenient  first  to  consider  a filter  which  does  not 
transmit  light  below  5,000  A.U.,  which  absorbs  the  whole 
of  the  ultra-violet  and  blue-violet,  but  does  not  absorb  any  green 


4,00  0 5,000  6,000  tooo 

Fig.  25.  Sharp-cut  Filter  on  Erythrosine  and  Panchromatic 

Plates 

or  any  red.  This  filter  will  be  a bright  yellow  in  colour,  yellow 
being,  as  we  have  seen,  made  up  of  green  light  and  red  light — 
that  is  to  say,  yellow  being  simply  an  absorption  of  blue.  Con- 
sider the  effedl  of  this  on  an  orthochromatic  plate  which  has 
^ of  its  sensitiveness  in  the  blue  and  ^ in  the  green.  The 
yellow  screen  will  remove  all  the  blue  light,  i.e.y  of  the  adtive 
light,  and  it  will  increase  the  required  exposure  40  times,  so 
that  it  is  what  we  term  a 40  times  screen. 

Now  consider  the  same  screen  to  be  used  with  the  W ratten 
panchromatic  plate.  With  this  plate  £ of  the  whole  sensitive- 
ness is  in  the  blue,  J-  in  the  red  and  green.  The  screen  will  then 
remove  J-  of  the  adtive  light,  leaving  only  -J-  to  adt;  it  will 

39 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

increase  the  exposure  8 times.  This  example  shows  at  once  the 
intimate  relation  between  the  plate  and  the  multiplying  factor 
of  a filter. 

Take  now  a filter  cutting  the  spedtrum  sharply  at  5,500. 
This  screen  will  be  bright  orange  in  colour.  It  transmits  all 
the  yellow-green,  orange,  and  red  light.  It  absorbs  the  blue- 
violet  and  blue-green  light,  /.*.,  adopting  our  convention  as  to 
the  division  of  the  spedtrum — it  transmits  the  red  and  half  the 
green,  and  absorbs  the  blue  and  half  the  green.  The  efFedt  of 
this  on  the  ordinary  orthochromatic  plate  is  to  remove  the  blue 
sensitiveness,  of  the  whole  sensitiveness  of  the  plate ; but 
inasmuch  as  this  plate  is  not  sensitive  to  the  blue-green,  and 
the  yellow-green  region  of  sensitiveness  which  represents  the 
other  Jg-  of  the  sensitiveness  of  the  plate  is  transmitted  by  the 
filter  undiminished,  the  filter  will  only  increase  the  exposure 


4,000  5.000  6.000  7000 

Fig.  26.  Sharp-cut  Orange  Filter  on  Erythrosine  and  Pan- 
chromatic Plates 

40  times,  being  the  same  increase  as  is  shown  by  the  former 
screen. 

On  the  panchromatic  plate,  however,  the  matter  is  different, 

of  the  sensitiveness  of  the  plate  is  in  the  blue,  and  is  removed 
by  the  filter,  ^ is  in  the  green,  and  half  of  this  is  removed  by 
the  filter;  so  that  the  sensitiveness  left  is  y1^,  due  to  the  un- 
diminished red-sensitiveness,  and  being  half  of  the  green 
sensitiveness — the  total  residual  sensitiveness,  therefore,  being 
fV  of  the  original  sensitiveness,  and  this  filter  will,  on  the 
Wratten  panchromatic  plate,  increase  the  necessary  exposure 
1 0§  times. 

Again,  consider  a filter  cutting  the  spedtrum  at  6,000 — that 
is,  transmitting  all  the  red,  but  absorbing  all  the  blue  and  all  the 
green.  The  ordinary  orthochromatic  plate  has  no  appreciable 
sensitiveness  in  the  red,  and  therefore  could  not  be  used  in 
pradtice  with  such  a screen.  The  Wratten  panchromatic  has 

40 


THE  FACTOR  OF  A SHARP-CUT  FILTER 


y1^  of  its  total  sensitiveness  in  the  red,  and  consequent!)  this  red 
filter  will,  on  that  plate,  be  a 16  times  screen. 

Let  us  now  examine  into  the  multiplying  factor  of  the  filter 
which  will  give  correct  reproduction  of  red,  green,  and  blue,  a* 
seen  by  the  eye.  We  have  assumed  in  all  these  figures  that,  in 
order  to  get  correct  reproduction,  the  sensitiveness  for  red,  green,, 
and  blue  should  be  equal ; that  is,  we  have  chosen  our  units 
with  that  condition  in  mind.  On  the  orthochromatic  plate  we 
have  no  red  sensitiveness,  but  the  nearest  approximation  to  cor- 
reCt  rendering  that  we  are  able  to  obtain  will  be  given  if  the 
green  and  blue  are  of  equal  intensities,  we  require  a sensitive- 
ness in  the  blue  equal  to  the  sensitiveness  in  the  green.  The 
sensitiveness  of  this  plate  in  the  green  is  ^ of  its  total  sensitive- 
ness, so  that  we  must  use  a screen  which  will  give  us  of  its 
total  sensitiveness,  y1^  being  in  the  green,  and  in  the  blue. 


4000  5,000  COOC  T,000 

Fig.  27.  Orthochromatic  Filter  on  Erythrosine  and  Pan- 
chromatic Plates 

That  is,  it  must  cut  off  38  of  the  39  parts  of  blue  sensitiveness 
which  the  plate  has,  and  the  screen  will  increase  the  exposure 
20  times. 

With  the  Wratten  panchromatic  plate  we  have  TV  of  the 
sensitiveness  in  the  red,  and  y1^  in  the  green,  consequently  we 
must  have  ^ in  the  blue;  that  is,  the  total  sensitiveness  will 
be  and  the  increase  of  exposure  required  by  the  filter  will 

be  5^  times.  This  filter  will  reduce  the  4 sensitiveness  of  the 
blue  to  y1^,  it  will  remove  of  the  blue  sensitiveness.  Two 

points  must  be  noted  here : 

First  that  the  panchromatic  plate  will  require  very  much  less 
exposure  to  correCf  it  fullv  than  will  the  orthochromatic  plate,, 
and  secondly  that,  not  only  is  less  exposure  required,  but  that 
a lighter  screen  is  necessary;  that  is  to  say,  in  the  one  case  we 
had  to  remove  all  but  of  the  blue,  but  in  the  other  j1^  of  the 
blue  was  left,  and  consequently  a screen  which  would  give  the 

4 1 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

maximum  correction  obtainable  on  an  ordinary  orthochromatic 
plate  will  over-correct  the  panchromatic  plate. 

When  working  by  artificial  light  (except  enclosed  arc  lamps) 
the  proportion  of  “ colour  ” sensitiveness  rises,  and  that  of 
“ violet  ” sensitiveness  falls,  and  these  alterations  greatly  affeCt 
the  multiplying  factors  of  filters,  as  well  as  the  relative  sensitive- 
ness of  orthochromatic  and  panchromatic  plates  as  compared 
with  ordinary  plates. 

With  incandescent  gas  almost  full  correction  upon  the  Wratten 
panchromatic  plate  is  obtained  by  the  use  of  the  Ki  filter. 

A point  of  some  interest,  which  is  occasionally  referred  to  in 
the  photographic  press,  is  the  multiplying  factor  of  two  filters 
used,  the  one  on  the  top  of  the  other.  It  is  often  put  as  follows: 
Suppose  we  have  two  filters — a three  times  filter  and  a five 
times  filter — how  much  will  they  increase  exposure  if  used 
together?  The  increase  can  be  found  neither  by  adding  nor 
multiplying  the  separate  faCtors  of  each  filter,  but  the  answer 
must  depend  entirely  on  the  nature  of  the  filters,  and  somewhat 
on  the  plate.  For  instance,  one  might  be  a deep  violet  filter  and 
the  other  a strong  yellow,  in  which  case  it  would  be  possible  for 
them  together  to  let  through  no  light  at  all!  On  the  other 
hand,  with  a panchromatic  plate,  if  one  were  a K3  filter  and 
the  other  a filter  in  depth  between  the  Ki  and  the  K2,  the 
effedl  would  be  negligible,  and  the  multiplying  faCtor  (and  cor- 
rection) of  the  combined  filter  would  be  the  same  as  that  of  the 
first  one  (K3)  used  alone. 

If  the  plate  is  to  be  taken  as  an  ordinary  orthochromatic 
plate  and  the  filters  are  clear  yellow  filters,  one  being  a Ki  filter 
and  the  other  a filter  between  Ki  and  K2,  the  combined 
multiplying  faCtor  would  be  about  7.  If  the  filters  were  brown 
glass  filters,  of  the  type  formerly  used,  the  combined  faCtor 
would  probably  be  about  10. 

While  this  paragraph  may  serve  to  inform  some  who  have 
puzzled  over  the  question,  it  is  not  to  be  taken  as  recommend- 
ing the  use  of  two  filters  together.  Such  a procedure  is  not  at 
all  desirable,  especially  on  optical  grounds. 

Contrast  Filters 

These  filters  differ  from  orthochromatic  filters  in  that  it  is 
not  desired  to  obtain  in  them  a gradually  increasing  absorption 
as  shown  in  fig.  23,  but  as  sharp  a transition  as  possible  between 
the  region  of  absorption  and  that  of  transmission. 

A red  contrast  filter  (such  as  the  w A ” filter),  for  instance, 

42 


THE  FACTOR  OF  A SHARP-CUT  FILTER 


when  examined  in  a speCtroscope  will  be  seen  to  give  a spectrum 
like  fig.  28  in  which  the  absorption  is  complete  up  to  the  point 
where  the  yellow-green  passes  rapidly  through  yellow  into 
orange,  and  at  this  point  the  absorption  falls  suddenly  to  almost 
nothing,  practically  all  the  orange  and  red  light  being  trans- 
mitted. 

To  find  the  faCtors  of  these  contrast  filters  we  require  to 
know  only  the  relative  sensitiveness  of  the  plate  to  the  part  of 
the  light  transmitted  compared  with  its  sensitiveness  to  the  part 
of  the  light  absorbed.  Assuming  that  a panchromatic  plate  has 
about  r*g-  of  its  total  sensitiveness  in  the  red  and  orange  por- 
tions of  the  speCtrum,  about  another  being  in  the  green, 
yellow-green,  and  blue-green  portions,  while  the  remaining 
f is  in  the  blue,  violet,  and  ultra-violet  regions,  the  factor  of 
the  “ A ” filter  is  consequently  16,  since  it  transmits  only  the 


COLOUR  Invisible  Limit  of  Violet  Blue  Blue-  Green  Yellow-  Orange  Red  Deep-  Limit  of 
TO  EYE  Ultra-Violet  Visibility  Green  Green  Red  Visibility 

Fig.  28.  Spectrum  transmitted  by  “A"  Filter 

red  and  orange  rays,  to  which  the  plate  has  ^ of  its  sensi- 
tiveness. 

A useful  strong  yellow  contrast  filter  is  the  “G”  filter. 
This  filter  absorbs  all  the  ultra-violet,  violet,  blue,  and  blue- 
green  light,  transmitting  the  remainder.  On  the  Wratten 
panchromatic  plate  it  has  a multiplying  factor  of  8,  with 
daylight. 

Other  contrast  filters  have  approximately  the  following 
faCtors  on  the  Wratten  panchromatic  plate: 


Filter. 

Colour. 

tJ  Multiplying 

Fa&or. 

A. 

Red. 

Tricolour  work,  Mahogany  Fur- 
niture, Cloud  photography 

16 

B. 

Green. 

Tricolour  work . 

16 

C 

Blue. 

Tricolour  work  . 

6 

E. 

Orange. 

T wo-colour  work,  Contrast  filter 

12 

F. 

Strong  Red. 

Copying  Blue  Prints,  Screenplate 
Analysis  .... 

43 

30 

“P”  Filter  “L”  Filter 


Fig.  29.  Colour  Chart  reproduced  through  various  Wratten 
Contrast  Filters 


i 

i 


4+ 


THE  FACTOR  OF  A SHARP-CUT  FILTER 


Filter. 

Colour.  Use. 

Multiplying 

Fa&oi 

G. 

Yellow.  Telephotography, 

Furniture, 

General  Contrast  . 

8 

L. 

Violet.  Screenplate  Analysis  . 

b 

N. 

Strong  Green.  Screenplate  Analysis  . 

.24 

P. 

Blue-Green.  Two-colour  work,Cop\ 

ingType- 

writing 

• ’ • 15 

R. 

Deep  Red.  Contrast  . 

. 80 

Aesculine.  Colourless.  Photography  or 

drawings 

containing  Chinese  White  . on  Process  Plate 
Infra-red  filter,  after  Professor  R.  W.  Wood 

on  the  SpeCtrum  Panchromatic  plate  3,000 

The  results  obtained  by  photographing  the  frontispiece 
through  these  contrast  filters  are  shown  in  fig.  29. 


Orthochromatic  Plate  con- 
taining its  own  filter 


<s 


Wratten  Panchromatic  Plate 
without  filter 


Fig. 


Beside  these  contrast  filters  special  u M ” filters  are  manu- 
factured for  use  in  photomicrographic  work,  and  these  are  par- 
ticularly recommended  in  conjunction  with  the  Wratten  “ M ” 
panchromatic  plate  for  photomicrography.  A description  of 
these  filters,  instructions  for  their  use,  multiplying  faCtors  for 
them  when  used  with  the  a M ” plate  for  daylight  and  various 
artificial  light  sources,  are  given  in  the  W ratten  booklet  u Photo- 
micrography.” 

With  reference  to  the  filter  faCtors  given  above,  it  must  be 
noted  that  these  faCtors  are  only  approximate,  and  constant 
endeavours  are  being  made  so  to  improve  the  colour  sensitive- 
ness of  the  Wratten  panchromatic  plate,  that  the  ratio  of  blue 
sensitiveness  to  green  and  red  sensitiveness  will  become  lower 
and  lower.  At  the  time  of  this  book’s  revision,  the  ratio  is 
considerably  lower,  the  red  sensitiveness  being  so  much  im- 

45 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


proved  that  the  factor  with  the  red  filter  is  not  sixteen,  but  ten. 
The  colour  sensitiveness  is  tested  for  ever)'  batch  of  plates,  and 
the  exaCt  factors  for  eight  filters  are  given  on  the  instructions 
enclosed  in  every  box  of  plates.  These  factors  must  be  taken 
and  not  the  merely  approximate  ones  given  in  this  book.  The 
two  illustrations  herewith  show  the  extreme  red  sensitiveness 
of  the  Wratten  panchromatic  plate.  The  one  is  taken  on  an 
orthochromatic  plate  containing  its  own  filter,  and  the  other  is 
on  the  Wratten  panchromatic  without  any  filter  whatever.  The 
flower  is  a red  anemone. 


46 


CHAPTER  VI 


THE  RENDERING  OF  COLOUR  CONTRASTS 

IT  should  be  clear  by  now  that  by  orthochromatic  photo- 
graphy we  intend  to  imply  the  use  of  a fully  colour- 
sensitive  plate,  such  as  the  Wratten  panchromatic,  combined 
with  a filter  of  necessary  strength,  to  give  approximately  the 
same  tone-rendering  as  that  seen  by  the  eye.  It  must  be  re- 
membered in  the  first  place  that,  to  the  eye,  objects  are  picked 
out  from  their  surroundings  by  contrast,  and  this  contrast  may 
be  of  two  kinds,  it  may  be  tone  contrast,  that  is  the  contrast  of 
light  and  shade,  or  it  may  be  colour  contrast.  In  the  case  of 
tone  contrast,  if  we  imagine  ourselves  to  be  dealing  with  a 
monochromatic  scene,  of  a colour  within  the  limits  of  sensitive- 
ness of  the  plate  used,  any  plate  will  render  tone  contrast  of 
considerable  range  as  seen  by  the  eye;  but  in  the  case  of  colour 
contrast  the  question  will  require  more  careful  thought.  Suppose 
that  we  have  two  objects,  the  one  contiguous  to  the  other,  and 
separated  from  each  other  to  the  eye  purely  by  their  colour 
contrast,  such  as  a green  field  containing  a patch  of  red.  The 
contrast  between  them  is  marked  to  the  eye,  although  the  tone 
contrast  is  very  nearly  nothing,  that  is  to  say,  the  two  are  of 
much  the  same  visual  luminosity.  If  we  photograph  them  upon 
an  ordinary  plate  both  are  black  to  it,  and  we  get  our  contrast 
represented  by  one  uniform  field  of  black;  the  colour  contrast 
has  disappeared,  and  we  have  a totally  unsatisfactory  rendering 
of  that  which  we  are  photographing.  If,  however,  we  photo- 
graph them  upon  a green  sensitive  plate,  then  the  green  will  be 
distinguished  from  the  red  as  brighter,  and  we  shall  get  a certain 
degree  of  contrast  of  a kind,  but  if  we  photograph  them  upon  a 
panchromatic  plate  with  a K3  screen,  so  that  we  get  a render- 
ing of  both  colours  in  their  true  luminosity  value  to  the  eye, 
the  contrast  again  disappears,  and  the  colours  are  represented  by 
a uniform  field  of  gray. 

What  then  must  we  do  to  obtain  a satisfactory  rendering  of 
this  colour  contrast?  Clearly  it  is  not  possible  to  render  the 

* 47 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

colour  contrast  accurately  in  monochrome,  so  long  as  we  retain 
the  rendering  of  correct  luminosity  values  for  our  colours,  and 
consequently  we  must  sacrifice  the  correct  rendering  of  either 
the  red  or  the  green.  If  we  use  a paler  filter  or  a green  filter, 
the  green  will  appear  the  brighter  and  the  red  the  darker;  if 
we  use  a deep  orange  filter,  the  red  will  be  brighter,  the  green 
darker;  and  which  we  shall  use  must  be  governed  by  circum- 
stances. As  a general  rule,  if  we  must  correct  wrongly  for  the 
rendering  of  colour  contrast,  it  is  usually  better  to  over-correct 
towards  the  red,  since  red  is  a strong  colour,  while  green  is  a 
weak.  For  example,  in  a field  of  corn  of  a deep  yellow  colour, 
we  may  have  poppies  standing  out  which  are  nearly  as  bright 
as  the  corn,  and  it  is  necessary  to  decide  whether  we  shall  render 
them  as  brighter  or  as  darker  than  the  corn.  Probably  on  an 
actual  measurement  of  luminosities  they  would  be  a little  darker 
than  the  corn,  but  remembering  the  way  in  which  the  strong 
red  attracts  the  eye,  it  would  seem  that  a more  faithful  render- 
ing would  be  given  by  over-corredting  and  rendering  the  poppies 
as  brighter  than  the  corn.  Again,  the  top  of  a yellow  straw  stack 
against  a deep  blue  sky  may  give  a result  with  perfect  ortho- 
chromatism where  the  haystack  is  indistinguishable  from  the 
background.  Here  again  it  is  probably  better  to  over-corredl, 
though  the  individual  worker  must  decide  for  himself.  A thing 
to  guard  against  always  is  the  danger  of  basing  one’s  considera- 
tion of  monotone  rendering  upon  photographs;  we  are  apt  to 
take  our  conception  as  to  the  tone  value  of  bright  green  grass, 
lor  instance,  from  photographs  which  invariably  have  shown  it 
as  too  dark,  if  not  black.  Frequently,  in  a spring  landscape,  the 
hedges  and  grass  are  almost  the  brightest  things  in  the  whole 
landscape,  and  they  should  clearly  be  rendered  as  light  grays; 
but  so  uniform  is  the  belief  among  photographers  that  grass  is 
black,  that  a rendering  as  light  gray  will  often  provoke  the 
comment  that  the  photograph  is  over-corredted. 

The  most  important  case  of  colour  contrast  occurs  in  the 
copying  of  pidlures,  and  for  this  purpose  Dr.  Mees  some  time 
ago  suggested  a special  method,  which  may  be  explained  here. 

Th  is  method  depends  upon  the  use  of  tricolour  filters,  the 
plate  being  exposed  first  through  one  filter  and  then  through 
another,  in  order  to  get  the  desired  colour-rendering.  It  is  first 
necessary  to  remove  a common  misconception,  which  one  fre- 
quently finds  repeated  in  text-books  and  the  technical  press, 
namely,  that  the  effedt  of  printing  from  the  three  tricolour 
negatives  on  one  piece  of  paper  would  be  to  give  an  orthochro- 
matic  result.  This  would  give  an  isochromatic  result,  that  is  to 

48 


RENDERING  OF  COLOUR  CON  ERASTS 


say,  one  in  which  all  colours  are  rendered  of  equal  strength, 
independently  of  their  visual  brightness;  this  results  in  an  excess 
of  brightness  in  the  red  and  blue,  especially  in  the  blue,  and 
insufficient  brightness  in  the  green,  the  whole  colour-rendering 
being  wrong.  Suppose  that  we  put  a set  of  filters  in  front  of 
our  lens,  fitted  in  a slide-past  holder,  so  that  we  can  expose  the 
plate  through  the  three  filters  in  succession  without  removing 
the  plate  or  dark  slide.  Then  we  may  give  an  exposure  through 
the  three  filters,  in  proportion  to  their  ratio  upon  that  plate. 
Supposing,  for  example,  that  we  have  a plate  and  a set  of 
filters,  such  that  the  blue  requires  6 times  the  normal  exposure, 
the  green  requires  12  times,  and  the  red  requires  18  times,  if 
we  give  through  the  blue  twice  the  normal  exposure,  the  plate 
will  be  \ exposed.  Now  give  through  the  green  4 times  the 
normal  exposure — the  plate  is  now  ^ exposed — and  now  super- 
pose on  this  an  exposure  through  the  red  screen  of  6 times  the 
normal  exposure;  we  have  now  a negative  combining  our  three 
colour  negatives  in  one;  but  it  will  not  be  correct  rendering  at 
all,  it  will  give  all  blues  much  too  light,  and  greens  too  dark, 
and  the  results  will  be  unsatisfactory.  With  the  Wratten  filters 
and  plate,  owing  to  the  faCt  that  the  green  transmits  a certain 
amount  of  blue,  correct  colour-rendering  is  obtained  by  giving 
H of  the  exposure  through  the  green  and  } through  the  red. 
Thus,  in  the  example  just  given,  where  the  ratio  of  exposures 
for  the  three  filters  was  6 — 12 — 18,  the  correCt  rendering 
would  be  obtained,  together  with  correCt  exposure,  by  giving 
about  8 times  the  normal  exposure  through  the  green  and 
6 times  through  the  red.  Since  this  proportion  of  the  mixed 
exposures  of  green  and  red  gives  a correCt  orthochromatic  result, 
we  can  exaggerate  red  or  green  by  increasing  the  exposure  of 
the  one  filter  and  diminishing  the  exposure  of  the  other.  For 
instance,  if  we  give  9 secs,  exposure  through  the  red,  and 
6 secs,  through  the  green,  we  shall  have  exaggerated  red  at 
the  expense  of  green;  on  the  other  hand,  if  we  give  10  secs, 
through  the  green  and  3 secs,  through  the  red,  we  shall  ex- 
aggerate greens  at  the  expense  of  reds.  If  we  want  to  diminish 
greens  altogether,  and  bring  up  reds  and  blues,  we  can  use  our 
red  filter  and  blue  filter,  and  so  obtain  the  rendering  that  we 
desire  by  altering  the  relative  exposures  through  the  three 
tricolour  filters. 

This  method  may  sound  rather  far-fetched,  but  as  a matter 
of  faCt  it  has  been  adopted  by  some  very  skilled  picture  copiers 
with  perfeCt  success.  A very  important  point  about  this  method 
is  that  all  the  while  one  is  working  one  knows  how  far  one  is 

49  d 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

from  correct  rendering,  so  that  instead  of  more  or  less  over-  or 
under-corredling  by  a screen  of  which  the  adtion  is  somewhat 
uncertain,  one  can  say  quite  definitely : “ I exaggerated  the  reds  in 
that  reproduction  50  per  cent.,  because  it  was  necessary  to  pick 
out  the  red  against  the  green  in  the  shadows  ” — a statement 
which  is  more  scientific  and  more  useful,  both  to  the  speaker 
and  hearer,  than  a statement  such  as:  UI  used  a rather  dark 
screen  for  that  in  order  to  get  over-corredtion.” 

A word  of  warning  is  necessary  here  as  to  the  quality  of  the 
filters  required  for  this.  It  will  be  seen  that  the  three  images 
are  literally  superposed  upon  one  another,  and  that  the  very 
smallest  shift  in  any  one  of  these  images  will  produce  a double 
image  in  the  result,  consequently  a much  higher  grade  of  filter 
is  required  than  for  ordinary  reproduction  purposes.  It  is  not 
sufficient  that  the  images  should  be  of  the  same  size,  but  they 
must  actually  fall  on  the  same  place  on  the  focussing  glass. 
This  can  only  be  accomplished  by  the  use  of  filters  cemented 
in  optical  flats  of  the  very  highest  quality,  or  else  by  the  use  of 
gelatine  film  alone.  It  will  save  disappointment  if  the  faCt  is 
emphasized  that  what  are  usually  known  as  “ first-class  cemented 
filters  ” will  not  do  for  this,  and  even  when  using  flats  care 
should  be  taken  always  to  insert  the  filters  the  same  way. 

Colour  Contrast  for  Special  Purposes 

The  type  of  colour  contrast  which  we  have  been  describing 
is  simply  a concession  from  orthochromatism,  in  order  to  enable 
us  to  some  extent  to  make  up  for  the  failure  of  monotone  when 
it  is  necessary  to  render  colour.  But  there  is  another  case  of  the 
photography  of  colour  contrast,  which  is  to  the  technical  worker 
of  as  great  if  not  greater  importance,  and  that  is  the  photography 
of  coloured  objects,  per  se , in  order  to  obtain  the  best  possible 
results,  generally  for  reproduction  purposes. 

General  Principles. — If  a colour  is  to  be  rendered  as  black  it 
must  be  photographed  in  its  absorption  band  (see  Chapter  I)  by 
light  which  is  of  such  a wave  length  that  it  is  completely 
absorbed  by  the  colour.  That  colour  then  appears  as  black  as  it 
can  be  made.  A useful  example  is  given  by  a photomicrograph 
of  a section *stained  with  eosine:  this  sedlion  is  pink;  if  it  is 
viewed  by  blue  light,  owing  to  the  fadt  that  eosine  does  not 
absorb  blue,  it  looks  comparatively  light.  By  green-blue  light 
of  a wave  length  about  5,000  to  5,400,  which  is  completely 
absorbed  by  eosine  (the  absorption-spedtrum  of  eosine  is  shown 
in  fig.  31),  the  sedtion  is  entirely  black,  as  is  shown  by  the  first 

50 


5,000  "5, +oo  5 >7°°  6,400 

Fig.  32.  W.  L.  of  Light 


51 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

block;  being  blocked  up  in  detail,  this  gives  the  maximum 
degree  of  contrast  (fig.  32).  Photographing  at  5,700,  on  the 
border  of  the  absorption  band,  we  get  a considerably  lessened 
contrast,  which  for  this  particular  sedtion  will  give  us  the  best 
result.  There  is  plenty  of  detail  in  the  section,  while  at  the 
same  time  the  contrast  is  sufficient  for  reproduction  purposes. 
Photographing  at  6,400  in  the  red,  and  in  the  light  which  is 
completely  transmitted  by  the  section,  the  photograph  has  no 
contrast,  is  very  flat,  and  results  are  useless.  So  that  for  the 
maximum  contrast  we  must  photograph  in  the  absorption  band. 

For  example,  it  is  sometimes  necessary  to  copy  a print  of 
which  the  paper  has  become  yellow  with  age.  An  ordinary  plate 
is  sensitive  only  to  the  ultra-violet,  violet,  and  blue  rays,  which 
are  more  or  less  absorbed  by  the  yellow  paper,  so  that  if  a 
negative  is  made  of  such  a print  on  an  ordinary  plate  the  repro- 
duction of  the  yellow  paper  will  appear  dark,  or,  in  any  case, 
dirty.  If  a colour-sensitive  plate  is  used  with  a yellow  contrast 
filter  the  yellow  stain  will  have  no  efteCt  and  will  fail  to  photo- 
graph. It  should  be  noted  that  the  yellow  filter  for  such  a pur- 
pose should  not  be  an  orthochromatic  filter  if  the  best  results 
are  required,  but  a much  stronger  filter,  such  as  the  W ratten 
“ G ” filter,  because  an  orthochromatic  filter  is  adjusted  to 
photograph  objeCts  in  their  relative  luminosities  as  seen  by  the 
eye,  and  if  the  yellow  stain  is  visible  to  the  eye  it  will  also 
photograph  through  an  orthochromatic  filter.  If  the  stain  be 
examined  through  the  strong  “ G ” filter  there  will  reach  the 
eye  no  light  which  is  not  yellow,  and  so  the  stain  will  not 
appear  different  from  the  white  ground. 

Fig.  33  shows  two  photographs  of  a platinotype  print  which 
had  been  splashed  with  yellow  dye  so  as  to  leave  a yellow  stain. 
In  the  upper  photograph,  taken  on  an  ordinary  plate,  the  stain 
appears  quite  black,  while  in  the  lower  one,  for  which  a pan- 
chromatic plate  has  been  used  with  a “ G ” filter,  the  stain  has 
entirely  disappeared,  only  a trace,  which  cannot  be  reproduced, 
being  visible  in  the  negative. 

Another  difficulty  is  sometimes  met  with  in  copying  prints 
due  to  the  faCt  that  the  prints  are  of  a brown  colour,  such  as  is 
given  by  sepia  platinotype,  carbon,  or  toned  bromide  prints. 
This  brown  colour  has  a very  much  stronger  absorption  for  the 
violet  light,  to  which  the  plate  is  sensitive,  than  for  the  yellow- 
green  and  orange  light,  which  represents  the  maximum  sensitive- 
ness of  the  eye,  and  consequently  such  prints  when  photo- 
graphed on  a 11011-colour  sensitive  plate  give  negatives  having 
far  too  much  contrast,  and  with  blocked  up  shadows,  and  it  will 

52 


On  Ordinary  Piate 


On  Panchromatic  Plate  with  “G”  Filter 
Fig.  33.  Yellow-stained  Platinotype  Print 


53 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

generally  be  found  that  no  increase  of  exposure  will  reproduce 
satisfactorily  such  photographs.  The  obvious  course  is  to  photo- 
graph them  as  the  eye  sees  them,  that  is  by  means  of  a fully 
correcting  filter  and  a panchromatic  plate. 

A difficult  task  without  the  proper  plate  and  filter  is  the 
photography  of  engineers’  and  architects’  blue  prints.  Ordinary 
orthochromatic  plates  with  yellow  filters  do  not  give  the  best 
results  with  such  a subjeCt  because  a great  deal  of  the  yellow- 
green  light  to  which  such  plates  are  sensitive  is  reflected  by  the 
blue,  and  in  order  to  obtain  really  first-rate  results  the  M A ” or 
u F”  filter  should  be  used  with  “Process  Panchromatic”  plates, 
thus  photographing  the  print  by  red  light  which  is  completely 
absorbed  by  the  blue  colour.  With  such  a plate  and  filter  the 
results  from  a blue  print  are  in  every  way  as  satisfactory  as 
could  be  obtained  from  a black  and  white  print  in  the  ordinary 
way  (fig.  34). 

Suppose,  to  take  another  example,  we  have  a sheet  of  type- 
writing, with  corrections  in  red  ink;  the  violet  typewriting 
absorbs  the  whole  of  the  orange  and  green,  the  red  ink  absorbs 
only  the  green.  If  we  photograph  through  the  green  filter,  u B,” 
of  the  tricolour  set,  we  shall  get  both  the  typewriting  and  the 
red  ink  completely  black,  and  therefore  the  greatest  contrast 
which  can  be  obtained  (fig.  35).  If,  on  the  other  hand,  we 
photograph  through  the  red  “ A ” filter,  the  typewriting  will  ap- 
pear plainly  visible,  but  the  red  ink  will  show  so  little  contrast  that 
it  easily  can  be  intensified  out  of  existence,  and  we  can  make  a 
reproduction  of  the  sheet  showing  the  typewriting  only 

(%•  36)- 

Most  commercial  work,  such  as  catalogue  illustrations  of 
carpets,  wall  papers,  linoleums,  china,  marble,  etc.,  can  best  be 
accomplished  by  the  aid  of  the  K3  filter  on  the  panchromatic 
plate,  but  occasionally  a red  or  green  filter  will  be  found  ex- 
tremely useful.  For  general  commercial  photography  the  follow- 
ing set  of  filters  will  be  found  to  cover  most  requirements: 
Ki,  K2,  K3,  tricolour  set  (red,  green,  and  blue),  strong  red 
(“  F ”),  and  strong  yellow  (M  G ”). 

For  one  branch  of  commercial  photography,  furniture  work, 
the  tricolour  red  “A”  and  the  yellow  “G”  filters  are  in- 
valuable; the  “A”  filter  used  with  the  panchromatic  plate 
giving  a splendid  rendering  of  the  grain  of  red  mahogany  such 
as  can  be  obtained  in  no  other  way.  For  satinwood  and  inlaid 
work  the  “ G ” filter  is  required,  so  that  for  furniture  photo- 
graphy a set  comprising  the  K3,  “ G,”  and  “A”  filters  should 
be  obtained. 


54 


( b ) Blue  print  photographed  on  Panchromatic  Plate  through  “A”  Filter 

Fig.  34 


55 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

Subjects  for  which  a correctly  orthochromatic  rendering  is 
particularly  desirable  are  postage  stamps  and  reproductions  of 
coloured  advertisements,  such  as  posters. 

In  copying  maps  a K3  filter  must  be  used  if  the  map  con- 


The  typewriting  is  in  violet  ink. 

The  script,  including  correction,  is  in  red  ink. 

6c 

In  photographing  typewriting,  a green  screen  must^uaed:  if  there 
are  any  red  ink  corrections,  the  green  screen  will  record  these  also 
4<nJCA/t^#  Cv  axcC 

^ 

Fig.  35.  Typewriting  and  Red  Ink  through  Green  “ B”  Filter 

tains  several  colours,  but  in  the  maps  which  more  often  come 
to  a commercial  photographer,  such  as  land  survey  maps,  a con- 
trast filter  is  frequently  required  to  accentuate  some  special 
colour  in  the  original. 

For  photographing  new  houses,  and  indeed-  most  modern 

The  typewriting  is  in  violet  ink 

The  script,  including  correction,  is  in  red  ink 


In  photographing  typewriting,  a green  screen  must  used:  if  there 
are  any  red  ink  corrections  the  green  screen  will  record  these  also ■ 


Fig.  36.  Typewriting  and  Red  Ink  through  Red  “A”  Filter 

architecture,  the  K3  or  “G”  filter  with  a panchromatic  plate  will 
give  admirable  results,  avoiding  the  sombre  tone  in  which  red 
bricks  are  too  often  rendered,  especially  in  dull  or  hazy  weather. 

By  the  application  of  this  principle,  viz.:  to  use  a filter  that 
absorbs  the  colour  which  is  to  be  rendered  as  black,  we  can  pick 
out,  in  faCt,  any  colour  from  a combination  of  colours,  and  in 
two,  three,  or  four  printings  obtain  a facsimile  result. 

56 


Whalebone  Seftion  photographed  for  Contrast 


57 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

The  second  principle  of  importance  is  that,  where  a uniformly- 
coloured  thing  is  to  be  photographed,  and  the  best  rendering  is 
to  be  obtained,  it  must  be  photographed  not  in  its  absorption 
band,  but  in  the  transmission  or  refledtion  region  of  the  colour. 
For  instance,  in  photographing  the  eosine-stained  section,  we 
get  the  greatest  contrast  by  photographing  in  the  absorption 
region  of  the  stain;  but  we  obtain  that  contrast  at  the  expense 
of  the  loss  of  detail  in  the  sedtion,  and  we  get  the  greatest 
detail  in  the  photograph  where  we  used  the  red  light.  Owing, 
however,  to  the  fadt  that  we  must  keep  contrast  against  the 
background  in  this  case,  our  best  final  result  was  a compromise 
between  contrast  and  detail,  obtained  by  photographing  on  the 
border  of  the  absorption  band.  A very  good  example,  however, 
of  the  use  of  light,  such  as  is  transmitted  by  the  stain,  is  shown 
by  the  two  photographs  of  a whalebone  sedtion,  which  are  re- 
produced here  from  the  little  book  on  u Photomicrography.” 
The  upper  one  shows  the  sedtion  photographed  for  contrast  by 
means  of  light  which  is  absorbed  by  it;  the  lower  one  shows 
the  same  sedtion  photographed  in  order  to  show  detail  bv  the 
light  which  it  transmits  (fig.  37). 

Perhaps  the  most  important  application  of  this  method 
occurs  in  the  photography  of  furniture,  where  the  results  are 
simply  surprising  to  the  uninitiated.  If  a piece  of  reddish 
mahogany  is  photographed  on  an  ordinary  plate,  no  trace  of 
grain  is  usually  visible.  The  photograph  is  made  by  blue  light 
to  which  both  the  red  darker  portions  and  the  yellow  light 
portions  are  black;  to  give  an  increased  exposure  simply  results 
in  the  photography  of  a plentiful  crop  of  normally  invisible 
scratches.  If,  however,  a panchromatic  plate  sensitive  to  the 
red  is  used,  with  an  orange  or,  better,  a red  filter,  the  results  are 
entirely  different;  the  scratches  disappear  and  the  grain  comes 
up  in  the  most  wonderful  way;  in  fadt,  so  startling  is  the  dif- 
ference, that  probably  many  readers  will  think  that  the  example 
shown  in  Fig.  38  is  faked,  but  if  they  try  the  experiment 
they  will  get  similar  results.  Many  examples  and  hints  on  this 
work  are  given  in  the  W ratten  booklet,  “The  Modern  Method 
of  Photographing  Furniture.” 

It  must  be  noted  that  for  this  purpose  the  plate  must  be  red 
sensitive,  as  red  mahogany  has  a strong  absorption  in  the 
greenish  yellow  of  the  ordinary  isochromatic  plate,  and  it  may 
be  well  to  remark  again  at  this  point,  as  at  the  beginning,  that 
the  whole  of  the  discussion  of  this  book  is  based  on  the  use  of 
plates  that  are  panchromatic.  Ordinary  and  ordinary  ortho- 
chromatic  plates  may  be  ruled  out  when  we  are  dealing  with 

58 


59 


On  Ordinary  Plate  On  W ratten  Panchromatic  Plate  through  Red  Screen 

Fig.  38 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

technical  work,  which  requires  us  to  work  in  any  region  of  the 
spectrum  which  may  be  necessitated  by  the  colour  of  our 
objeCt. 

A useful  example  of  this  same  principle  of  photographing  in 
the  coloured  light  which  is  reflected  from  the  object,  is  given 
by  the  photography  of  prints  for  reproduction  purposes.  We 
have  already  referred  to  the  case  of  brown  prints,  and  in  the 
same  way  a red  silver  print,  when  being  photographed  on  wet 
plates  for  half-tone  work,  is  well  known  as  a most  difficult  sub- 
ject, requiring  usually  a large  amount  of  fine  etching.  These, 
and  other  cases  of  the  same  kind,  can  be  dealt  with  very  easily 
by  using  Wratten  panchromatic  plates  with  a medium  filter. 
The  prints  then  become  as  easy  to  copy  as  any  black-and-white 
subjeCts. 


f 


60 


CHAPTER  VII 


PORTRAITURE 

IN  no  branch  of  photography  is  the  reproduction  of  coloured 
objedts  in  correCt  monochrome  of  greater  importance  than 
in  portraiture,  and  in  no  branch  is  it  in  greater  danger  of  being 
ignored.  The  flesh  tints,  with  which  portrait  photographers 
are  mainly  concerned,  are  chiefly  of  a reddish  or  yellowish 
nature,  while  the  yellow  and  brown  shades  of  the  hair  and  the 
variety  of  the  eye-colours,  apart  altogether  from  the  clothing, 
cause  every  sitter  to  present  a distinCt  problem  in  colour  repro- 
duction. Earnest  efforts  to  meet  this  problem  have  been  to  a 
large  extent  discouraged  by  the  assistance  which  the  retoucher 
can  give  in  correcting  the  errors  introduced  by  incorreCt  colour- 
rendering. Retouching,  however,  is  always  a dubious  remedy, 
and  though  expert  artists  may  make  good  use  of  it,  it  leads  to 
many  pitfalls  for  most  workers.  We  have  only  to  look  at  a 
lantern  slide,  made  from  a retouched  portrait  and  projected  upon 
the  screen,  to  realize  how  difficult  it  is  for  retouching  to  be 
applied  satisfactorily.  When  making  an  ordinary  enlargement 
it  is  often  necessary  to  remove  the  whole  of  the  retouching,  so 
badly  does  it  show.  Even  if  the  retoucher  were  able  to  lighten 
satisfactorily  those  parts  of  the  flesh  which  the  ordinary  plate 
has  failed  to  render  with  sufficient  density,  he  would  still 
be  unable  to  darken  correCtly  these  parts  where  the  excess  of 
sensitiveness  to  blue  and  violet  has  produced  too  heavy  a deposit 
in  the  negative. 

The  rendering  of  colour  in  portraiture  is  governed  by  the 
same  laws  that  govern  the  reproduction  of  colour  in  other  sub- 
jects. Those  who  have  studied  the  earlier  chapters  of  this  book 
will  realize  that  photographing  with  ordinary  plates  is  equivalent 
to  photographing  by  blue-violet  light,  to  which  alone  such  plates 
are  sensitive.  As  blue-violet  light  is  absorbed  by  flesh  tints,  the 
use  of  it  produces  an  accentuation  of  contrast  similar  to  that 
which  always  follows  when  any  coloured  objeCt  is  viewed  or 
photographed  by  light  which  is  selectively  absorbed  by  it.  (See 
Chapter  V.) 

61 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

Consequently  portrait  negatives  taken  upon  ordinary  plates 
always  show  more  contrast  than  the  subjects  appear  to  the  eye. 
To  overcome  this  difficulty  the  subject  may  be  either  lighted 
with  a softer,  flatter  light  than  would  otherwise  be  used,  or  the 
negative  may  be  over-exposed.  But  in  either  case  accuracy  of 
tone-rendering  in  all  the  portions  of  the  picture  which  are  not 
yellow  or  red  is  lost,  and,  to  put  it  as  it  appears  without  analysis, 
the  “ lights  ” are  degraded. 


{a)  On  Ordinary  Plate 


( b ) On  Panchromatic  Plate  with  K3  Screen 
Fig.  39 


Not  only  is  this  general  accentuation  of  contrast  produced, 
but  also  the  reproduction  of  the  skin  itself,  which  is  really  the 
fundamental  work  of  the  artist,  suffers  greatly  from  the 
contrast  effeCt  introduced  by  the  use  of  plates  insensitive  to 
red.  Fig.  39  will  possibly  make  this  clear.  The  photograph 
of  the  bar  of  Sunlight  soap  is  enlarged  from  a lithographic 
advertisement  in  which  the  bar  appears  to  be  very  faintly  mottled 
in  many  colours,  mostly  brown  and  red.  The  grain  is  very 
small,  and  these  brown  and  red  spots  being  not  only  small,  but 
alike  in  hue,  are  nearly  invisible  to  the  eye;  so  that  in  the 

62 


PORTRAITURE 


original  it  appears  smooth.  When  it  is  examined  by  violet  light, 
however,  the  violet  light  is  completely  absorbed  by  the  yellow, 
brown,  and  red  spots,  and  consequently,  as  was  explained  in  our 
discussion  of  colour  contrast,  the  contrast  between  these  and 
the  background  is  increased  greatly.  So  that  when  the  original 
is  photographed  on  an  ordinary  plate  a high  degree  of  contrast  is 
produced,  and  the  photograph  looks  very  mottled.  Photographed 
upon  a panchromatic  plate  with  a K3  filter,  we  obtain  an 
accurate  reproduction  of  the  lithograph,  giving  the  same  degree 
of  smoothness  as  was  apparent  to  the  eye.  This  shows  at  once 
how  any  uneven  stipple,  such  as  this  grain  exhibits,  produces 
this  mottled,  and  to  some  extent  woolly,  appearance  by  colour 
contrast. 

This  exaCt  effeCI  is  reproduced  in  photographing  the  skin. 
The  surface  of  the  skin  is  completely  heterogeneous.  The 
small  blood-vessels  which  cover  it,  and  the  pathological  changes, 
which  the  failure  of  the  pores  of  the  skin  to  do  their  work  per- 
fectly produce  with  increase  of  age,  make  a sort  of  stipple  of 
reddish  spots  and  streaks  over  the  whole  surface  of  the  skin. 

This  is  very  easily  seen  by  examining  any  portion  of  the 
skin  under  the  mercury  vapour  lamp.  The  light  from  the 
mercury  vapour  lamp  consists  for  essential  purposes  of  a mix- 
ture of  green  light  and  violet  light,  no  red  being  present.  The 
small  red  spots  of  the  skin  absorb  very  completely  the  green 
light  from  the  mercury  vapour  lamp,  but  refleCt  to  a consider- 
able degree  the  violet  ray,  thus  producing  a general  background 
of  white  light  stippled  over  with  violet  spots.  The  same  effeCI 
can  be  seen  to  a less  extent  by  examining  the  skin  through  a 
green  filter,  when  the  mottling  becomes  much  more  marked 
than  it  is  to  the  eye,  but  appears  of  course  black,  and  not 
violet. 

The  same  unfortunate  contrast  which  produces  this  effeCt 
on  the  skin  also  accentuates  wrinkles.  Wrinkles  are  generally 
lined  by  small  networks  of  capillaries,  which  have  the  practical 
effeCt  of  producing  a red  line  on  each  side  of  the  wrinkle. 
This,  photographing  black  on  the  ordinary  plate,  greatly  accen- 
tuates the  depth  of  the  wrinkle.  The  wrinkle  at  the  side  of  the 
eyes  is  often  deep  red  in  colour,  and  consequently  prints  too 
dark  when  photographed  by  any  but  red  sensitive  plates,  while 
the  delicate  shadows  under  the  lower  eyelid  which  give  round- 
ness and  shape  to  the  eye  are  seldom  truthfully  rendered  with 
the  ordinary  plate,  though  when  they  do  appear  the  retoucher 
generally  removes  them. 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

pletely  absorbing  the  violet,  appear  as  black  spots.  The  two 
portraits  of  the  lady  (Fig.  40)  were  taken,  the  one  on  a pan- 
chromatic plate  with  a K3  filter,  and  the  other  on  an  ordinary 
plate,  exposures  being  given  which  render  the  neutral  detail  the 
same  in  the  two  cases.  The  accentuation  of  freckles  and 
wrinkles  with  the  ordinary  plate  is  of  course  very  marked,  but 
on  observing  the  skin  texture,  the  point  as  to  general  smooth- 
ness will  also  be  realized,  though  this  is  somewhat  lost  in  the 
reduced  reproduction. 

An  instance  of  the  opposite  fault  to  that  of  insufficiently 
recording  red  or  yellow  markings  upon  the  plate,  and  thus 
accentuating  their  appearance,  is  shown  by  the  way  in  which 
any  bluish  tinge  produces  too  great  a density  and  prints  too 
light.  The  best  example  of  this  is  possibly  the  mouth.  The 
upper  lip  being  usually  rather  more  in  shadow  and  reflecting 
less  ultra-violet,  appears  black  in  a photograph  taken  upon  an 
ordinary  plate,  but  the  blue,  violet,  and  ultra-violet  has  its  full 
effeCt  from  the  lower  lip,  and  that,  therefore,  reproduces  un- 
naturally white,  so  that  the  mouth  is  more  often  like  a quarter 
of  an  orange  than  a rosebud. 

When  dealing  with  the  question  of  colour  contrast,  it  was 
shown  that  while  photographing  in  the  absorption  band  of  a 
colour  produces  contrast,  photographing  in  the  transmission 
band  will  produce  an  entire  lack  of  contrast.  Clearly,  then, 
the  deeper  the  filter  we  use,  the  smoother  the  result  will  become, 
and,  indeed,  if  a portrait  be  taken  through  the  red  filter  used 
in  tricolour  photography,  the  failure  of  contrast  is  so  complete 
that  the  skin  appears  absolutely  smooth,  giving  quite  a false 
impression.  There  is,  however,  an  application  of  this  general 
principle  of  photography  in  the  transmission  band  which  we 
must  next  consider,  and  that  is  in  the  question  of  the  hair. 

This  subjeCl  is  of  great  importance  to  portraitists,  because  of 
the  extreme  difficulty  in  satisfactorily  retouching  hair. 

Brown,  golden,  or  red  hair  is  always  difficult  to  photograph, 
tending  in  the  darker  shades  to  produce  black  masses  without 
detail.  Golden  hair  is  usually  met  by  using  a dark  background 
(with  a little  paint  on  the  back  of  the  negative,  as  a correspondent 
ingenuously  remarks),  but  this  imposes  a needless  limitation,  and 
will  not  meet  the  not  infrequent  case  of  a light  golden  brown 
moustache.  Moreover,  hair  of  these  colours,  even  when  exposure 
and  lighting  have  produced  a satisfactory  tint,  will  always  show 
absence  of  detail  in  the  shadows.  This  is,  of  course,  a parallel 
instance  to  the  harsh  contrast  obtained  when  copying  a warm 
brown  or  red  print  upon  an  ordinary  plate. 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

It  may  be  as  well  to  mention  that  even  when  panchromatic 
plates  are  used,  and  the  resultant  negatives  need  no  retouching 
in  the  ordinary  sense  of  the  term,  the  work  of  the  artist  is  not 
finished.  Indeed  it  is  only  when  the  negative  is  reasonably 
accurate  that  the  retoucher  can  do  his  best  work.  People  with 
snub  noses,  large  ears,  crooked  teeth,  crow’s  feet,  or  gray  hairs, 
do  not  want  to  appear  as  they  are,  few  of  us  do;  but  a retoucher 
should  be  an  artist  who  improves  the  picture  while  preserving 
the  spirit  of  the  likeness,  and  not  a mechanic  who  has  to  atone 
for  the  faults  of  the  tools. 

There  is  no  need  to  dwell  at  length  upon  the  effect  of 
orthochromatic  rendering  upon  the  clothing  of  the  sitter.  To 
those  who  have  read  carefully  the  earlier  portion  of  this  book 
the  advantages  will  be  obvious,  and  is  well  shown  in  Fig.  40. 
A point  which  may  appeal  to  the  professional  photographer  is 
that  he  will  be  able  to  free  his  sitter  from  any  restraints  as  to  the 
clothes  which  shall  be  worn.  It  is  by  no  means  an  infrequent 
thing  for  a sitter  to  have  a favourite  costume,  and  to  be  dis- 
appointed at  the  difficulty  which  it  presents  to  the  operator. 

But  one  point  more  may  be  mentioned,  those  photographers 
who  tint  their  photographs  will  find  that  a panchromatic  ren- 
dering is  of  the  very  greatest  importance  as  enabling  them  to 
avoid  masses  of  black  where  it  is  most  important  that  they 
should  have  little  deposit  in  their  print,  in  order  to  get  bright 
colours.  To  get  a satisfa&ory  coloured  reproduction  of  a scarlet 
tunic,  for  instance,  it  is  necessary  that  the  negative  shall  have 
a good  deposit  of  silver  where  the  tunic  is,  so  that  the  colour 
shall  not  be  applied  to  a mass  of  black  in  the  print. 

Exposure. — The  use  of  filters  in  portraiture  raises  obvious 
difficulties  as  to  exposure.  Red  sensitive  plates  can  be  obtained 
which  are  of  the  same  speed  as  extra  rapid  plates,  quite  as  fast 
as  those  generally  used  for  studio  work.  For  ordinary  purposes, 
a Ki  filter  will  be  deep  enough,  and  this  will  increase  the 
exposure  by  half  as  much  again,  or  if  a little  more  exposure 
can  be  afforded,  then  the  KlF,  requiring  twice  normal  exposure, 
will  be  found  the  best  all-round  filter  to  use.  For  difficult, 
badly  freckled  subjeds,  or  such  a case  as  the  scarlet  tunic,  it 
will  be  necessary  to  use  a K2  filter.  Here  the  exposure  will  be 
three  to  four  times  that  to  which  the  worker  is  accustomed. 
Each  worker  will  know  for  himself  whether  he  can  manage 
this.  At  the  same  time  it  may  be  pointed  out  that  the  gain  in 
rendering  is  so  considerable,  that  it  will  make  the  best  of  any 
sacrifice  of  accuracy  in  the  lighting  if  that  cannot  be  avoided. 
Photography  in  ordinary  rooms  is  often  made  far  more  difficult 

66 


PORTRAITURE 


by  the  w unsuitable  ” colour  of  the  surroundings,  a difficulty 
which  is,  of  course,  eliminated  by  truly  orthochromatic  pro- 
cedure. Even  when  the  exposure  is  increased  by  the  use  of 
filters,  it  will  be  very  much  shorter  than  it  was  in  the  early 
days  of  photography,  when  wet  plates  were  used.  Then  an 
exposure  of  five  seconds  was  considered  unusually  short,  and 
although  we  have  no  wish  to  return  to  the  days  of  head-rests, 
yet,  if  we  except  young  children,  there  are  few  people  who  cannot 
keep  still  in  any  position  suggestive  of  repose  for  a much  longer 
period  than  five  seconds. 

Artificial  Light  Sources. — Artificial  light  sources  contain  as  a 
rule  much  more  red  and  green  light  than  daylight.  If  we  have 
two  sources  in  which  the  blue-violet  portions  of  the  spectrum 
between  4,000  and  5,000  A.U.  are  equal  in  intensity,  one  being 
daylight,  then,  if  the  other  be  acetylene  or  metallic  filament 
eledtric,  the  latter  will  give  out  seven  times  as  much  green 
and  twenty  times  as  much  red  light  as  daylight.  Incandescent 
gas  gives  about  twelve  times  as  much  green  and  from  fifteen  to 
twenty  times  as  much  red.  Enclosed  arcs  are  similar  to  day- 
light, but  if  used  with  red  or  yellow  flame  carbons,  the  pro- 
portion is  similar  to  acetylene.  Open  arcs  have  about  one  and 
a half  times  as  much  green,  twice  as  much  red. 

Since  we  know  the  relative  sensitiveness  to  blue,  green,  and 
red  of  the  plates  (Chapter  II),  we  can  construdf  the  following 
approximate  table : 

Light  source  compared  with  daylight,  taking  amount  of  blue- 
violet  light  as  the  same  in  the  two  sources. 

Relative  sensitiveness  of  the  two  plates  as  compared  with  an 
ordinary  plate  of  the  same  sensitiveness  to  daylight. 


Green 

Reel 

Light  Source. 

Pro- 

Pro- 

portion. 

portion 

Oil  or  carbon  filament 

eledtric  . 

20 

100 

Acetylene  or  metallic 
filament  (Osram)  . 

7 

20 

Incandescent  gas 

12 

20 

Yellow  flame  arc 

10 

20 

Open  arc  . 

1^ 

2 

Sensi- 

tiveness 

of 

Ery- 

throsine 

Plate. 


Sensi- 

tiveness 

of 

W ratten 
Pan- 
chromatic 
Plate. 


2 8 


3 

4 
4 

1^ 


67 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

It  follows  from  this  that,  owing  to  the  great  sensitiveness 
of  the  panchromatic  plate  to  the  yellow  artificial  light  sources, 
portraiture  by  their  aid  becomes  easy,  and  evening  portraiture 
thus  becomes  quite  practicable.  Street  work  at  night,  and  theatre 
photography,  become  also  much  easier  if  red  sensitive  plates 
are  used.  For  incandescent  gas  no  filter  is  used,  the  colour  of 
the  light  ensuring  sufficient  correction,  but  if  arcs  are  used 
(with  flaming  carbons)  a K2  filter  should  be  used,  owing  to 
the  very  strong  ultra-violet  and  violet  carbon  bands  which  must 
be  absorbed. 

The  recently  introduced  half- watt  nitrogen  filled  incan- 
descent eleCtric  lamps  are  exceptionally  suited  for  portraiture, 
since  they  enable  a much  more  powerful  artificial  light  to  be 
used  without  proportionate  increase  of  expense.  Their  light  is 
whiter  than  that  from  the  ordinary  metallic  filament  lamp,  and 
very  fairly  corrected  portraits  can  be  taken  by  the  half-watt 
lamp  on  the  Wratten  panchromatic  plate  with  the  KiF  filter. 

If  flashlight  is  used  for  portraiture,  it  will  be  found  that 
better  results  are  obtained  on  the  Wratten  panchromatic  plate 
without  any  filter  than  are  possible  on  any  ordinary  plate,  and 
a “ K ” filter  will  enable  very  full  corredion  to  be  obtained. 

The  Mercury  vapour  lamp  is  not  suitable  for  work  with  pan- 
chromatic plates.  It  does  not  emit  any  appreciable  amount  of 
red  light,  and  its  value  for  photographic  purposes  depends 
entirely  upon  the  violet  light  which  it  gives  out. 


68 


CHAPTER  VIII 


LANDSCAPE  PHOTOGRAPHY 

^T^HE  application  of  the  principles,  which  have  been  set  down 
X in  the  earlier  chapters  of  this  book,  to  the  photography  of 
landscapes,  presents  difficulties  of  which  most  workers  are  only 
too  well  aware.  The  discussions  which  follow  papers  on 
c<  Orthochromatism  ” in  photographic  societies  usually  turn  on 
these  difficulties,  and  the  variety  of  confli&ing  opinions  ex- 
pressed should  be  sufficient  warning  to  prevent  any  writer  from 
too  dogmatically  stating  what  should  be  done. 

To  M.  Andre  Callier,  a Belgian  worker  who  is  both  a first- 
rate  landscape  photographer  and  a scientific  investigator  of  great 
knowledge,  we  are  indebted  for  the  framework  of  this  chapter, 
and  for  many  of  the  points  with  which  it  deals  as  well  as  for  the 
Alpine  photographs  which  he  has  allowed  us  to  reproduce  (figs. 
41  and  42). 

Landscape  photography  presents  several  features  which  en- 
tirely distinguish  it  from  those  branches  of  work  with  which 
the  rest  of  this  book  is  concerned.  In  the  first  place,  landscapes 
display,  as  a rule,  in  our  northern  climates  a less  marked  scale 
of  contrast  than  the  subjects  with  which  we  are  accustomed  to 
deal  in  the  studio.  At  the  same  time,  however,  the  sky  is 
usually  of  much  greater  intensity  than  any  other  portion  of  the 
gradation  scale,  and  it  follows  that,  in  order  to  obtain  detail  in 
the  shadows  (seen  by  the  eye  because  of  the  expansion  of  the 
iris),  it  is  often  necessary  to  over-expose  the  sky. 

This  over-exposure,  which  destroys  differences  in  intensity 
which  are  perceived  by  the  eye  (clouds  for  instance),  can  be 
removed  by  the  use  of  contrast  colour  filters  which,  by  absorbing 
the  sky  light,  seem , in  certain  cases,  to  lengthen  the  scale  of  in- 
tensities which  the  plate  is  capable  of  rendering. 

It  is  desirable  to  point  out  that,  if  such  deep  filters  be  em- 
ployed, it  is  absolutely  necessary  that  the  exposure  should  be 
ample. 

Insufficient  exposure  will  result  in  a thin  sky  in  the  negative, 

69 


Fig.  41.  Telephoto,  Wratten  Colour-Sensitive  Plate 


I 


Fig.  42.  Wratten  Colour-Sensitive  Plate 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

and  in  general  hardness  in  the  foreground,  and  the  resulting 
pidure  will  give  a general  impression  of  over-correflioti. 

True  over-corredion  in  landscape  work  is  probably  very 
rare;  it  may,  of  course,  be  caused  by  the  deliberate  use  of  very 
strong  yellow  filters  (and  on  this  point  it  may  be  well  to  add  a 
reminder  that,  as  explained  in  Chapter  IV,  screens  which  are 
necessary  with  a slightly  colour-sensitive  plate  may  over-corred 
a panchromatic  plate),  but  as  a general  rule  the  appearance  of 
over-corredion  is  caused  by  under-exposure. 

Here  it  should  be  pointed  out  that  the  fador  given  by  the 
makers  of  exposure  meters  and  calculators,  in  reference  to  sea 
and  sky  subjeds  and  distant  landscapes,  must  be  used  with 
caution.  Thus  it  is  usually  stated  that  exposure  found  neces- 
sary for  average  landscape  is  all  that  is  required  for  sea  and  sky, 
but  this  is  only  true  when  no  filter  is  used ; if  a filter  is  put  on 
the  lens  cutting  out  all  the  ultra-violet  and  some  of  the  violet  and 
blue,  it  will  no  longer  be  true,  and  a negative  made  with,  say, 
a fador  of  rV  will  be  under-exposed  and  have  the  appearance  of 
over-corredion.  On  the  other  hand,  too  much  exposure  may 
be  given,  and  so  the  contrast  desired  entirely  flattened  out. 

That  is  to  say,  not  only  should  a suitable  filter  be  used,  but  an 
exposure  be  given  that  is  corred  for  the  degree  of  contrast 
required. 

The  aim  of  many  landscape  photographers  is  simply  to  get 
the  clouds  and  landscape  on  the  same  plate.  Comparatively 
small  corredion  is  sufficient  to  accomplish  this,  and  hence  the 
majority  of  such  photographers  use  a very  light  filter,  such  as 
the  Ki,  which,  with  a panchromatic  plate,  will  give  enough 
corredion  for  the  purpose  given  above,  and  at  the  same  time 
enable  the  camera  to  be  used  in  the  hand. 

But  those  workers  to  whom  truth  of  tone  is  of  the  first  im- 
portance will  desire  to  use  filters  of  greater  depth,  so  that  the 
colour  values  of  the  foreground  shall  be  corredly  translated  into 
monochrome. 

Many  such  workers  use  comparatively  deep  filters  with  plates 
sensitive  to  the  yellow-green,  but  not  to  red,  arguing  that  few 
landscapes  contain  red,  or  even  yellow,  and  that  the  greens  can 
be  satisfadorily  rendered  by  a green  sensitive  plate.  On  this 
point  the  following  quotation  from  a letter  from  M.  Callier 
may  be  of  interest : 

“ It  is  necessary  to  insist  upon  the  kind  of  orthochromatic 
plates  which  may  be  used.  In  spite  of  the  enormous  progress  real- 
ised by  plates  of  the  ervthrosine  type,  such  plates  show  a grave  de- 
fed  in  their  lack  of  sensitiveness  near  W.L.  5,000  ” (see  fig.  1 lb). 

72 


LANDSCAPE  PHOTOGRAPHY 


44  Usually  this  defied!  is  not  of  much  importance,  but  there 
are  certain  cases  where  it  becomes  a great  disadvantage.  This 
is  so  in  landscapes,  for  example,  which  contain  both  open 
meadows  and  pine  trees.  If  such  subjects  are  photographed  by 
means  of  plates  of  the  erythrosine  type  (especially  with  a filter, 
if  there  is  also  distance  to  be  rendered),  there  will  be  obtained 
in  the  negative  a greatly  exaggerated  contrast  between  the 
densities  of  the  meadows  and  of  the  pine  trees.  The  green  re- 
flected by  the  meadows  corresponds  to  the  maximum  of  sensi- 
tiveness of  an  erythrosine  plate,  while  that  coming  from  the 
pines  falls  exaCtly  into  the  gap  of  sensitiveness.  The  only 
method  of  obviating  this  is  to  use  a really  compensating  filter — 
that  is,  a filter  which  absorbs  the  violet  and  ultra-violet,  but 
which  also  has  an  absorption  about  the  region  5,600  correspond- 
ing to  the  maximum  of  sensitiveness  of  an  erythrosine  plate. 
Unfortunately,  the  increase  of  exposure  required  by  such  a 
filter  is  very  great. 

“From  this  standpoint  the  new  isocyanine  sensitisers  repre- 
sent a great  advance  over  erythrosine,  and  the  fad!  that  the  plates 
so  sensitized  are  also  sensitive  to  the  orange  and  red  constitutes 
a second  advantage  whenever  red  enters  into  a landscape.” 

An  important  fadtor  in  landscape  photography  which  does 
not  enter  into  studio  work  is  the  presence  suspended  in  the  air 
of  water  particles  which,  when  of  large  size,  condense  into  mist. 
It  is  well  known  that,  if  open  landscapes  are  being  photo- 
graphed, a very  slight  amount  of  mist  results  in  flat  negatives 
unless  strongly  corrected  orthochromatic  plates  be  used. 

The  reason  seems  to  be  that  the  suspended  particles  of  water 
vapour  which  are  transparent  for  the  longer  waves  of  light,  and, 
therefore,  only  affed!  vision  slightly,  ad!  as  a very  turbid  medium 
for  the  deep  violet  and  ultra-violet  waves,  scattering  them,  and 
producing  much  the  efFedt  that  would  be  seen  if  one  were  to  try 
and  look  through  a sheet  of  finely  ground  glass. 

As  the  water  vapour  condenses,  its  selection  of  the  longer 
wave  lengths  increases;  a fog,  for  instance,  will  absorb  the  blue 
and  green  rays  from  the  light  of  an  arc-lamp,  but  will  permit 
the  red  to  pass  in  greater  measure,  so  that  at  a little  distance 
the  lamp  will  appear  red. 

It  seems  probable  that  the  scattering  efFedt  of  mist  near  the 
ground  is  at  a maximum  in  the  ultra-violet,  and  that  this  scatter- 
ing decreases  as  we  pass  towards  the  red.  In  addition,  when  the 
sky  is  blue  the  mist  refledts  this  light,  and  appears  blue  from 
that  cause. 

In  order  to  remove  this  increased  effect  of  mist  in  the  nega- 

73 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

tive,  as  compared  with  the  effect  seen  by  the  eye,  we  must 
absorb  the  scattered  ultra-violet  and  violet  light  before  it  reaches 
the  plate  by  means  of  a filter.  It  is  to  be  noted  that,  to  be 
effective,  this  filter  must  absorb  the  ultra-violet  as  completely  as 
possible,  and  that  filters,  such  as  the  Wratten  u K.  ” filters,  are, 
therefore,  preferable  to  even  much  deeper  filters  made  of  other 
yellow  dyes  which  transmit  ultra-violet  light. 

The  removal  of  the  scattering  effect  of  mist  will  progressively 
increase  as  we  remove  the  violet,  blue,  and  greenish  blue,  by 
means  of  deeper  and  deeper  filters,  so  that,  if  strong,  sharp-cut 
filters  be  used,  the  air  will  appear  too  transparent — that  is,  there 
will  be  a loss  of  a atmosphere.”  It  is,  therefore,  important  that 
the  filter  should  be  of  gradual  cut,  corresponding  in  curve  to  the 
sensitiveness  of  the  eye,  and  that  sharp-cut,  strong  filters  should 
be  avoided.  In  telephoto  work,  however,  the  mist  intervening 
in  the  great  aerial  distances  between  the  lens  and  the  objedf  to 
be  photographed  is  a very  serious  and  real  difficulty,  and  a 
strong  contrast  filter,  such  as  Wratten  “G”  filter,  is  a great 
advantage.  Many  telephoto  workers  who  are  troubled  by  the 
flatness  and  fogginess  of  their  negatives  would  gain  much  by  the 
use — first,  of  a satisfadtory  lens  hood  cutting  off  all  light  not 
required;  and,  secondly,  of  a strong  contrast  filter. 

In  exceptional  cases  even  a red  filter  may  be  used  with  ad- 
vantage. Thus  some  photographs  have  been  made  of  the  Crystal 
Palace  from  Croydon,  a distance  of  about  four  miles,  in  which 
an  ordinary  plate  allowed  the  mist  completely  to  obliterate 
the  Palace.  With  the  Wratten  panchromatic  and  K3,  the  Palace 
was  photographed  as  the  eye  saw  it,  but  with  the  deep  red  UF  ” 
filter  the  Palace  was  very  much  plainer,  though,  of  course,  the 
colours  in  the  foreground  and  intervening  distance  were  over- 
corredted.  It  is  sometimes  stated  that  a process  plate  is  better 
for  rendering  distance,  but  there  is  no  advantage  in  this  where 
the  improved  rendering  is  only  to  be  obtained  by  eliminating 
the  effect  of  the  mist,  as  the  process  plate  is  just  as  susceptible 
to  the  ultra-violet  and  violet  rays  scattered  by  the  mist  as  are 
other  ordinary  plates. 

It  may  be  repeated  that  the  filter  used  for  telephoto  work 
must  either  be  plain  uncemented  gelatine,  or  must  be  cemented 
in  the  verv  best  optical  Flats.  The  great  equivalent  focal 
lengths  of  the  lenses  employed  will  not  permit  of  the  use  of 
ordinary  filters  if  the  best  definition  is  to  be  obtained.  The 
most  convenient  method  of  fitting  the  filter  is  usually  as  a cap 
on  the  back  of  the  negative  lens,  inside  the  camera,  which  posi- 
tion enables  one  to  employ  the  smallest  possible  screen. 

74 


LANDSCAPE  PHOTOGRAPHY 


While  dealing  with  telephoto  work,  it  may  be  pointed  out 
that  most  landscape  workers  could  take  a hint  from  the  tele- 
photographer with  regard  to  hoods. 

Modern  anastigmatic  lenses  are  made  to  work  at  such  great 
angles  that  they  are  seldom  fitted  with  hoods,  and  the  inevitable 
result  is  flatness  due  to  fog,  caused  by  the  light  scattered  in 
the  camera.  Landscape  workers,  who  do  not,  or  at  any  rate 
should  not,  employ  wide-angle  lenses,  should  fit  one  or  more 
hoods  to  their  lenses,  and  they  will  at  once  see  the  gain  in  their 
negatives. 

Assuming  the  plate  to  be  properly  developed  in  a safe  dark 
room  then  flat  and  foggy  negatives  are  due  to 

(1)  Scattered  light,  removed  by  a proper  hood; 

(2)  Mist,  removed  by  a proper  plate  and  filter  ; 
and  rarely  (3)  Over-exposure. 

Alpine  work  presents  a few  special  difficulties.  Great  dis- 
tances are  continually  occurring  in  consequence  of  the  purity 
of  atmosphere,  and  the  chief  difficulty  consists  in  retaining 
correct  gradation  between  the  skv  and  the  snow-clad  peaks  out- 
lined against  it. 

The  light  of  the  sky  is  due  to  the  numberless  dust  and  water 
particles  suspended  in  the  upper  air.  The  greater  reflecting 
power  of  these  small  particles  for  violet  and  ultra-violet  light 
causes  the  sky  colour  to  be  blue,  and  as  we  ascend  higher  into 
the  air  the  particles  decrease  in  size,  and  the  sky  reflection  be- 
comes less,  so  that  the  colour  becomes  a deep  blue,  and  at  very 
great  heights  the  sky  is  nearly  black.  (An  alternative  suggested 
by  Prof.  Spring  is  possible,  namely,  that  the  colour  of  air,  and 
especially  the  upper  air,  containing  much  ozone,  is  blue,  and 
that  the  upper  air  absorbs  the  red  light  from  the  white  light 
reflected  from  suspended  particles.) 

If,  therefore,  a deep  or  even  medium  filter  is  used,  it  may 
happen  that  the  sky  light  may  be  cut  out  too  completely,  and 
the  sky  will  appear  too  dark,  with  the  intensely  white  snow 
showing  in  great  contrast  against  it.  It  is  of  course  true  that 
this  is  to  a great  extent  also  the  effeCt  to  the  eye,  but  an  entirely 
truthful  rendering  may  be  displeasing  when  the  charm  of  the 
sky  colour  is  removed,  and  the  effeCt  is  certainly  exaggerated  if 
the  filter  used  is  of  too  sharp  cut. 

For  ordinary  Alpine  work,  a Wratten  panchromatic  plate  with 
a Ki  filter  is  of  sufficient  depth  to  render  both  sky  and  pines  satis- 
factorily against  the  snow,  but  if  there  is  much  colour  in  the  near 
foreground  then  a K2  may  be  advisable.  For  Alpine  telephoto 

75 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

work,  a K2  filter  is  necessary  to  remove  the  haze,  a “ G ” filter 
being  too  strong  if  the  distances  are  free  from  fog. 

The  nature  of  the  refledlion  of  the  sky  also  gives  us  a clue 
as  to  the  best  means  of  rendering  cloud  forms.  It  is  clear  that 
the  rendering  of  cloud  form  depends  on  causing  the  cloud  to 
have  the  maximum  effedt  on  the  plate  when  contrasted  with 
the  light  reflected  from  the  sky.  Since  the  light  refledted  from 
the  clouds  is  white,  while  that  from  the  sky  contains  a lesser 
proportion  of  the  longer  wave  lengths,  it  is  clear  that  the  deeper 
the  filter  the  greater  will  be  the  contrast.  Thus  the  use  of  an 
ordinary  plate,  photographing  by  means  of  the  ultra-violet  and 
violet  light,  will  usually  obliterate  the  contrast,  unless  the 
clouds  be  very  strong.  Using  a panchromatic  plate  and  a K3 
filter  we  shall  obtain  the  same  degree  of  contrast  as  that  which 
is  seen  by  the  eye. 

With  the  strong  yellow  u G 99  filter  this  contrast  will  be  exag- 
gerated, while  with  the  tricolour  “ A ” filter  we  get  a very  high 
degree  of  contrast,  making  this  filter  probably  the  most  useftil 
one  for  the  record  of  faint  cloud  forms.  By  the  use  of  an  even 
deeper  filter, such  as  is  obtained  by  using  the“D”  and  “G”  filters 
together — with  a special  plate  such  as  the  “ Wratten  spedtrum 
panchromatic  ” — we  can  photograph  near  the  limit  of  the  visible 
red,  and  owing  to  the  small  proportion  of  this  light  refledled  by 
the  sky,  can  record  wisps  of  vapour  which  are  barely  visible  to 
the  eye. 

It  is  quite  possible  to  photograph  by  means  of  the  infra-red 
rays,  and  Professor  R.  W.  Wood,  of  John  Hopkins  university, 
has  published  some  remarkable  results.  For  such  work  we  can 
supply  the  Wratten  infra-red  filter  as  used  by  Professor  Wood. 
It  must  be  used  in  conjunction  with  the  Wratten  spedtrum 
panchromatic,  as  the  ordinary  panchromatic  is  not  sufficiently 
sensitive  to  the  infra-red.  The  exposure  required  on  a land- 
scape in  bright  sunlight  will  be  from  two  to  five  minutes. 


76 


CHAPTER  IX 


THE  PHOTOGRAPHY  OF  COLOURED  OBJECTS  FOR 
REPRODUCTION 

HE  photographer  for  reproduction  will  have  many  objeCts 


to  photograph  similar  to  those  which  have  previously  been 
dealt  with,  and  the  procedure  will,  in  the  main,  be  similar.  It 
is  desirable  to  consider  for  what  process  the  negative  is  required 
before  deciding  on  its  character.  If  it  is  to  be  merely  an  ordin- 
ary photographic  copy  in  monochrome  of  a coloured  objeCt, 
then  the  usual  rules  apply — a negative  of  good  contrast  for 
carbon,  as  free  as  possible  from  fog  for  platinum,  of  softer  gra- 
dation for  bromide,  and  so  on.  If  the  negative  is  required  for 
mechanical  printing,  then  we  must  know  the  process, 
whether  for  a surface  process,  such  as  lithography  and  collo- 
type; intaglio,  such  as  photogravure;  or  relief,  such  as  half- 
tone, in  order  to  develop  to  a suitable  contrast. 

Photography  is  sometimes  brought  to  the  aid  of  pure  chromo- 
lithography, when  the  reproduction  is  to  be  of  a different 
size  from  the  original,  or  if  it  is  framed  and  cannot  be  removed 
from  the  frame.  The  first  thing  necessary  in  the  process  is  to 
make  a key  tracing,  which  is  an  outline  made  on  transparent 
transfer  paper,  of  every  patch  of  colour  showing  variation  from 
the  next  patch.  This  has  to  be  transferred  on  to  all  the  stones 
used  to  build  up  the  coloured  picture  in  order  that  the  litho- 
graphic draughtsman  may  know  exaCtly  where  to  put  his  work, 
so  that  in  the  end  the  “register”  or  fit  of  the  various  colours  is 
perfect.  If  now  a photograph  must  be  used  instead  of  the 
original  itself,  it  is  obvious  that  a negative  must  be  made  which 
best  distinguishes  the  variations  of  colour,  and  that  the  correct- 
ness or  otherwise  of  their  translation  into  monochrome  is  of  no 
importance  whatever.  The  plate  used  and  filter  chosen  must 
therefore  depend  entirely  on  the  character  of  the  subjeCt,  having 
this  end  in  view.  Viewing  the  original  through  a number  of 
differently  coloured  filters  will  frequently  be  of  assistance.  It 
will  sometimes  be  found  that  an  ordinary  plate,  without  any 


77 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

filter  at  all,  will  distinguish  the  patches  of  colour  better  than 
any  other  procedure.  With  regard  to  collotype  and  photo- 
gravure, which  require  ordinary  negatives,  colour-sensitive  plates 
with  contrast  or  compensating  filters  should  be  used,  as  the 
particular  subjeCt  requires. 

Now,  with  regard  to  photo-lithography  and  relief  processes, 
such  as  ordinary  half-tone,  in  which  the  final  result  is  to  be  a 
surface  broken  up  into  grain,  the  grain  is  nearly  always  required 
in  the  negative  from  which  the  copying  on  to  the  printing  sur- 
face is  done.  Consequently  it  is  an  economy  if  the  negative, 
which  gives  us  the  colour  record,  can  also  be  split  up  into  grain 
at  the  same  time.  To  work  in  this  manner  is  called  the 
“ direCt  method,”  and  the  Wratten  process  panchromatic  plates 
are  specially  designed  for  the  purpose.  Whether  it  is  possible 
to  work  “direCt”  or  not  depends  on  the  amount  of  contrast 
contained  in  the  original. 

In  a grained  negative  the  illusion  of  tone  is  secured,  not  by  vary- 
ing density  (/.*.,  thickness  of  deposit  of  silver)  as  in  an  ordinary 
continuous  tone  negative,  but  by  deposits  of  silver  in  the  form 
of  spots  of  very  great  opacity,  but  of  varying  size.  Places  where 
the  spots  are  very  small  and  there  is  much  clear  glass  will  repre- 
sent shadow,  those  where  the  spots  are  very  large  and  there  is  little 
clear  glass  will  represent  the  high  lights,  and  proportionately  for 
other  tones ; the  size  of  the  dots  everywhere  corresponding  with 
the  amount  of  light  reflected  from  the  original.  Except  in  certain 
cases,  when  the  so-called  “high-light”  negatives  are  required, 
any  given  area  in  the  high  lights  must  contain  some  transparent 
spaces,  and  in  the  shadows  must  contain  some  points  of  dense 
silver.  But  it  is  obvious  that,  if  there  is  a considerable  amount 
of  contrast,  it  will  be  impossible  to  fulfil  the  necessary  conditions, 
because  before  any  points  of  silver  have  impressed  themselves 
in  the  shadows  of  the  negative,  the  dots  in  the  high  lights  will 
have  received  so  much  exposure  as  to  make  them  completely 
cover  up  all  the  transparent  spaces,  and  that  part  will  no  longer 
serve  as  a grained  negative.  So  that  only  certain  objects  or 
originals  are  suitable  for  reproduction  by  the  “direCt”  manner, 
which  consists  in  placing  the  objeCt  in  front  of  the  camera, 
illuminating  it,  and  photographing  with  the  half-tone  cross  line 
or  irregular  grain  screen  in  front  of  the  plate.  The  aCtual 
details  of  this  work  would  be  out  of  place  here;  further  details 
can  be  obtained  from  the  Wratten  booklet  on  “ Reproduction 
Work  with  Dry  Plates.”  It  may,  however,  be  well  to  point  out 
that  originals  with  heavier  contrasts  than  about  sixteen  to  one 
should  not  be  attempted  by  the  “direCt”  process;  that  is,  if  the 

78 


FOR  REPRODUCTION 


light  reflected  from  the  shadows  is  taken  as  one,  then  that  re- 
flected from  the  high  lights  should  not  be  more  than  sixteen  times 
as  much.  It  is  true  that  heavier  contrasts* are  often  done,  and 
made  to  pass  by  the  waving  of  white  paper  in  front  of  the  original 
during  the  exposure,  a practice  known  technically  as  “ flash- 
ing,” but,  beyond  a small  amount,  this  pra&ice  is  very  strongly 
to  be  deprecated,  as  it  is  ruinous  to  detail  and  gradation,  and 
plates  and  filters  must  not  be  blamed  if  the  results  appear  flat 
and  unsharp  when  this  is  done. 

With  subjeCls  of  heavy  contrast,  resort  must  be  had  to  the 
“indireCt”  process.  In  this  a negative  is  made  in  the  ordinary 
way,  but  the  exposure  and  development  are  so  arranged  that, 
while  all  the  detail  is  secured,  the  density  of  silver  deposit  is 
restricted,  so  that  the  negative  does  not  exceed  a certain  range 
of  contrast.  From  the  negative  a positive  is  made,  either  on 
paper  or,  preferably,  on  a slow  dry  plate  (*.*.,  a transparency). 
This,  while  having  all  the  details  of  the  original,  will  have  the 
contrasts  compressed  so  that  they  are  within  the  limits  possible 
to  the  half-tone  process,  and  a grained  negative  can  now  easily 
be  made  in  the  usual  manner,  either  on  wet  collodion  or  on 
another  dry  plate. 

As  examples  of  common  subjects  having  heavy  contrasts,  we 
may  take  most  solid  objeCIs  such  as  articles  of  furniture,  carpets, 
etc. ; though  some  articles  for  catalogue  illustration,  such,  for 
instance,  as  sweets  or  packets  of  soap,  may  verv  well  be  done 
direCI.  Many  oil  paintings,  especially  old  ones,  are  better 
reproduced  by  the  indireCl  process. 

The  next  thing  to  be  considered  is  the  style  of  reproduction. 
If  for  monochrome  printing,  then  the  principles  already  out- 
lined in  previous  chapters  must  be  applied.  If  colour  contrast  is 
required,  then  a filter  must  be  used  absorbing  that  colour  which 
it  is  appropriate  to  render  darkest,  and  a plate  sensitive  to  the 
colours  that  are  not  required  to  print.  If,  on  the  other  hand, 
correct  luminosity  values  are  wanted,  then  a K2  or  K3  filter 
should  be  used  with  a Wratten  panchromatic  plate.  In  general 
it  will  be  found  that,  for  colour  work  in  a reproduction  studio, 
the  panchromatic  plate  will  be  most  suitable;  for  “ diredt  ” 
grain  negatives,  the  Wratten  process  panchromatic. 

In  the  case  of  an  original,  such  as  a brown  or  sepia  carbon 
print,  that  by  reason  of  its  colour  is  difficult  or  impossible  to 
do  on  an  ordinary  or  wet  collodion  plate,  it  is  generally  suffi- 
cient to  use  a Wratten  panchromatic  or  process  panchromatic 
plate  without  any  filter;  though  to  secure  the  utmost  possible 
detail  it  may  sometimes  be  necessary  to  use  a Ki  filter. 

79 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

The  simplest  cases  of  colour  reproduction  are  presented  by 
stained  MSS.,  typewriting,  cheques,  maps,  and  so  forth.  These 
nearly  all  require  thd  use  of  contrast  filters.  A full  discussion 
on  the  correct  procedure  will  be  found  in  Chapter  VI. 

Next  come  subjeCls  to  be  reproduced  in  two  colours.  Some- 
times these  are  drawn  in  two  colours,  sometimes  in  more;  in 
either  case  it  is  desirable  to  know  what  coloured  inks  are  to  be 
used  in  the  reproduction.  Filters  are  then  selected  so  that  the 
light  reflected  from  the  parts  of  the  original  which  it  is  desired 
to  print  in  one  of  the  inks  shall  be  absorbed  and  the  negative 
be  transparent  there.  Thus,  supposing  we  have  a crayon  draw- 
ing of  a lady’s  head  in  pink  and  yellow,  we  want  a green  filter 
to  absorb  the  pink  and  allow  the  yellow  to  pass,  and  a blue- 
violet  to  pass  the  pink  and  absorb  the  yellow.  Many  good 
colour-efFeCts  may  be  obtained  in  two  printings  when  the  two 
inks  together  make  a black.  Any  two  inks  of  colour  com- 
plementary to  each  other  will  give  a black  and  scale  of  grays, 
as  well  as  the  two  colours  separately.  Thus,  an  orange-red 
and  a greenish-blue  will  give  those  two  colours  and  black;  a 
green  and  pink,  the  same;  an  ultramarine  blue  and  yellow 
the  same.  'Phis  method  can  be  applied,  also,  when  we  only 
have  one  colour  and  black;  for  example,  a red  and  black.  The 
use  of  the  red  filter,  a A,”  and  a panchromatic  plate  will  permit 
only  the  black  to  be  photographed ; the  second  negative  is 
made  with  a blue  filter,  and  will  give  us  both  the  red  and 
black.  Now  if  this  be  printed  in  an  ink  imitating  the  red  of 
the  drawing,  the  black  can  be  printed  in  black,  or  in  an  ink 
which  on  the  top  of  the  red  will  make  a black. 

Another  method  is  to  make  a positive  from  the  negative 
taken  through  the  red  filter;  now  register  upon  this  the  nega- 
tive taken  through  the  blue  filter.  This  latter  negative  records 
both  blacks  and  reds  as  clear  spaces,  while  the  positive  records 
only  reds  as  clear  spaces,  so  that  the  two  together  are  equiva- 
lent to  a negative,  in  which  the  red  of  the  subject  is  re- 
presented by  clear  spaces.  The  black  negative  is,  of  course, 
taken  through  the  red  filter.  If  we  have  several  colours  and 
black  the  procedure  is  more  difficult,  and  it  is  sometimes 
troublesome  to  extract  the  black  if  the  colours  are  at  all  dark. 
A filter  should  be  selected  that  does  not  completely  cut  out 
any  colour  present,  but  that  transmits  most  freely  the  deepest 
colour.  Sometimes  it  is  most  convenient  to  expose  the  same 
plate  for  a portion  of  the  time  through  each  of  the  filters  of 
a tricolour  set,  but  the  filters  have  to  be  made  with  best  optic- 
allv  worked  glass,  as  otherwise  the  resulting  image  will  appear 

80 


FOR  REPRODUCTION 


doubled.  Sometimes,  on  the  other  hand,  no  filter  at  all  is  neces- 
sary, or  at  most  a light  yellow  filter,  and  a sufficiently  long 
exposure  will  give  the  black  alone  on  the  negative. 

It  is  unnecessary  to  deal  with  the  three-colour  process  here, 
as  a separate  chapter  is  devoted  to  that,  and  the  methods  are 
exaCtly  the  same  in  a reproduction  studio,  except  that  arc 
lamps  are  frequently  substituted  for  daylight.  These,  however, 
of  whatever  type,  do  not  cause  any  other  adjustment  than  cor- 
rections in  the  exposure  ratio  of  the  filters,  which  will  not  be 
the  same  as  for  daylight. 

It  is  yet  far  from  being  realized  how  much  help  could  be 
obtained  from  intelligent  photography  in  photo-lithography,  for 
the  use  of  the  camera,  with  suitable  plates  and  filters,  could 
save  much  of  the  lithographic  draughtsman’s  work.  We  have 
seen  beautiful  reproductions  of  water-colours  by  photo-litho- 
graphy where  the  lithographer  has  had  a photographic  basis 
on  the  stone,  and  these  were  produced  in  days,  when  without 
the  photographic  work  weeks  would  have  been  required. 
Though  these  reproductions  have  been  made  in  from  five  to 
ten  printings,  the  perfection  of  the  plates  and  filters  now  ob- 
tainable, if  used  with  knowledge,  would  enable  excellent  repro- 
ductions to  be  made  with  very  few  printings,  and  this  question 
is  worthy  of  the  serious  attention  of  all  chromo-lithographers. 


CHAPTER  X 


THREE-COLOUR  PHOTOGRAPHY 
(i)  The  Additive  Process 

THREE-COLOUR  photography  is  based  on  the  fadt,  first 
discovered  by  Clerk  Maxwell  in  i860,  that  all  colours  can 
be  matched  by  a mixture  of  three  primary  colours — a red,  a 
pure  green,  and  a blue — if  the  proportion  of  these  constituent 
colours  be  rightly  chosen.  By  an  apparatus  which  he  termed 
the  “ Colour  Box,”  Maxwell  determined  the  exadt  position 
in  the  spedtrum  of  these  three  primary  colours,  and  also  the 
proportion  in  which,  at  each  point  of  the  spedtrum,  they  must 
be  mixed  in  order  to  reproduce  to  the  eye  the  sensation  pro- 
duced by  the  light  of  the  spedtrum  itself.  The  intensities  of 
the  primaries  at  each  point  form  three  overlapping  spedtral 
curves,  which  are  shown  in  fig.  43.  If,  now,  we  construdt 
a set  of  three  filters  which,  when  used  with  a suitable  plate, 
will  give  these  curves,  we  can  obtain  in  the  spedtograph  a 
set  of  negatives  in  which  the  opacities  at  every  point  of  the 
spedtrum  are  proportional  to  the  light  intensities  for  each  primary. 
If  we  make  from  these  negatives  transparencies,  and  projedt 
the  three  transparencies  so  that  they  converge  upon  one  screen 
by  means  of  approximately  monochromatic  lights  exadtly  cor- 
responding with  the  primaries,  we  shall  reproduce  the  spedtrum  ; 
or  rather,  we  should  reproduce  the  spedtrum  if  our  plates  would 
rigidly  translate  light  intensity  into  the  equivalent  opacity. 
This  they  will  not  quite  do;  at  the  same  time  the  reprodudtion 
is  very  good.  This  result,  however,  can  only  be  obtained  if 
the  corredt  exposure  is  given  to  the  spedtrum.  If  a number  of 
varying  exposures  to  the  spedtrum  be  given,  it  will  be  found 
that  the  reprodudtion  will  vary,  not  in  intensity  only,  but  also 
in  colour.  The  reason  can  easily  be  seen  by  referring  to  the 
diagram  (fig.  43)  showing  the  curves  of  the  filters.  Take,  for 
instance,  the  point  6,100,  which  should  be  recorded  to  a certain 
extent  in  the  red  filter,  and  to  about  } of  this  extent  in  the 

82 


THREE-COLOUR  PHOTOGRAPHY 


green  filter,  and  should  therefore  be  reproduced  by  the  whole 
of  the  red  light,  and  by  J of  the  green,  thus  appearing  orange. 
With  the  shortest  exposure,  however,  this  will  record  only  in  the 
red  filter,  and  not  at  all  in  the  green;  that  is,  it  will  be  pro- 
jected as  a faint  but  pure  red.  On  the  other  hand,  with  great 
exposure  the  plate  will  be  quite  opaque  in  both  the  red  and 
the  green  negatives,  and  in  reproduction  we  shall  have  a bright 
yellow  caused  by  the  mixture  of  all  the  red  with  all  the  green. 
The  same  is  true  of  nearly  all  points  in  the  speCtrum.  With 
over-exposure  the  region  about  5,200,  for  instance,  which  should 
be  pure  green,  will  be  recorded  to  some  extent  in  all  three 
filters,  and  will  reproduce  as  a greenish  Jwhite.  This  variation 


of  colour  with  exposure  is  of  great  importance  in  practical 
photography,  as  whatever  variation  there  is  in  intensity,  the 
true  hue  must  always  be  reproduced.  It  is  consequently  neces- 
sary that  the  absorption  curves  of  the  filters  should  be  as  abrupt 
as  possible,  and,  as  a matter  of  faCt,  suitable  filters  are  shown 


in  fig.  44. 

Such  filters  will  not  satisfactorily  reproduce  the  speCtrum; 
they  will  divide  it  into  five  sharp  and  abrupt  regions,  h irst, 
the  region  extending  down  to  6,000,  which  has  been  recorded 
in  the  red  filter  only,  and  which  is  therefore  reproduced  as  pure 
red.  Secondly,  the  narrow  region  between  5,900  and  6,000, 
which  has  been  recorded  through  both  the  red  and  the  green 
filters,  and  which,  therefore,  reproduces  as  a mixture  ot  red 
and  green  light,  that  is,  as  yellow.  I hirdly,  the  region  between 
5,900  and  5,000,  which  is  recorded  only  in  the  green  filter,  and 

82 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

reproduces  as  green.  Fourthly,  the  region  between  4,800  and 

5.000,  recorded  in  both  the  green  and  blue  filters,  and  repro- 
duced as  blue-green.  Fifthly,  the  region  between  4,800  and 

4.000,  recorded  only  in  the  blue  filter,  and  reproduced  as  blue. 
It  will  be  noticed  that  this  failure  is  only  in  the  reproduction 

of  pure  colours  other  than  the  projecting  colours  themselves 
and  their  simple  mixtures.  In  natural  objeCts  such  pure  colours 
do  not  occur.  In  the  speCtrum,  for  instance,  a green  of  wave 
length  5,600  is  a yellowish  green,  while  a green  of  wave  length 
5,200  is  a bluish  green.  Both  will  reproduce  alike  with  sharp- 
cut  filters,  since  both  will  be  recorded  only  in  the  green  filter. 
The  yellowish  green  of  any  objeCt  will,  however,  be  recorded 
fully  in  the  green  filter,  and  to  a less  extent  in  the  red  filter, 
while  it  will  only  record  to  a very  slight  extent  in  the  blue 
filter.  It  will,  therefore,  reproduce  as  a yellowish  green.  A 


f 


4060  3,000  6,000  7000 

Fig.  44.  Theoretical  Tricolour  Filters 

natural  blue-green,  on  the  other  hand,  will  be  recorded  to  but 
a very  small  extent  through  the  red  filter,  and  to  a much  larger 
extent  through  the  blue  filter,  so  that  it  will  reproduce  as  a 
blue-green.  The  illustrations  show  the  absorption  curves  of 
dyes  made  up  to  match  exaCtly  to  the  eye  the  spedral  regions 
(fig-  45)- 

The  transparencies  made  from  the  negatives  taken  through 
the  three  filters  can  be  projected,  either  by  means  of  a triple 
lantern  or  by  such  a device  as  the  photochromoscope.  The 
filters  to  be  used  for  projection  are  somewhat  different  both 
from  the  taking  filters  and  also  from  the  original  narrow  bands 
which  we  have  hitherto  assumed. 

In  projection  with  the  triple  lantern,  especially,  the  great 
difficulty  is  to  obtain  sufficient  light,  and  this  difficulty  at  once 
prohibits  any  approach  to  monochromatic  illumination.  In 
order  to  get  bright  colours,  it  is  necessary  that  the  absorptions 
of  the  proje&ing  filters  should  be  abrupt,  and  that  they  should 

84 


1 


THREE-COLOUR  PHOTOGRAPHY 


not  appreciably  overlap  one  another.  Inasmuch  as  the  taking 
filters  do  somewhat  overlap,  it  is  better  to  use  a different  set 
for  purposes  of  projection.  The  red  projecting  filter  should  be 
a true,  strong  red,  not  an  orange — that  is  to  say,  it  must  not 
pass  any  light  of  shorter  wave  length  than  6,000  A.U.  The 
green  should  be  a pure  green,  not  transmitting  any  blue,  and 
extending  from  6,000  to  5,000  A.U.  It  would  seem  that  the 
taking  blue  filter  should  be  suitable  also  for  projecting;  if,  how- 
ever, the  triple  lantern  be  set  up,  and  the  above-described  red 


SPECTRUM  SHOWING 
PURE  COLOURS  WITH 
TRACE  OF  FILTER 
CURVES 


CURVE  OF  NATURAL 
YELLOW  - GREEN 
MATCHING  PURE 
LIGHT  OF  W.L.  5,600 


CURVE  OF  NATURAL 
BLUE-GREEN  MATCH- 
ING PURE  LIGHT  OF 
W.L.  5,200 


and  green  filters  be  used,  together  with  standard  blue,  it  will 
probably  be  found  that  the  field  is  yellow.  This  is  due  to  the 
faCt  that  dyes  absorb  some  of  the  light  which  they  are  sup- 
posed to  transmit,  and  the  proportion  which  they  absorb  de- 
pends on  the  dye  (see  Chapter  I).  Owing  to  the  fad  that 
absorption  bands  are  nearly  always  sharper  towards  the  red  end 
of  the  speCtrum  than  towards  the  blue  end,  a red  filter  will 
absorb  very  little  red  light.  Our  red  filter  absorbs,  at  6,400,  16 
per  cent,  of  the  incident  light.  A green  filter  absorbs  much  more 
green  light;  at  5,900  our  green  filter  absorbs  74*3  per  cent., 
transmitting  only  about  a quarter.  At  5,400  it  transmits  about 

85 


FlO.  45 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

half  the  incident  light.  But  a blue  filter  absorbs  very  much 
blue  light.  At  4,800  our  blue  filter  transmits  about  one-fifth 
of  the  light,  and  the  same  proportion  at  4,500.  This  absorp- 
tion of  useful  light  by  the  blue  filter  is  not  a disadvantage 
in  photographic  work,  because,  even  with  the  best  red-sensitive 
plates,  the  exposure  through  the  red  filter  is  greater  than  that 
through  the  blue  filter,  and  if  the  blue  filter  were  lighter,  the 
effect  on  the  total  exposure  would  be  inconsiderable,  while  the 
exposure  through  the  blue  filter  would  be  so  short  that  it  would 
be  more  difficult  to  give  it  accurately.  But  this  absorption  of 
the  blue  light  by  the  blue  filter  is  a serious  disadvantage  if  the 
filter  is  used  in  tricolour  projection  and  it  is  therefore  neces- 
sary to  use  the  brightest  blue  filter  which  can  be  obtained. 

This  additive  synthesis,  as  it  is  called,  by  means  of  a photo- 
chromoscope or  triple  lantern,  is  much  the  easiest  process  of 
three-colour  photography  to  work,  and  gives  also  the  best  and 
most  accurate  reproduction  of  coloured  objeCts.  A modification 
which  has  recently  greatly  developed  is  the  screen-plate  method 
of  colour  photography.  In  this  method,  suggested  by  Ducos  du 
Hauron,  and  first  carried  out  by  Joly,  a glass  plate  is  divided 
into  a number  of  coloured  elements,  these  elements  being  so 
small  that  they  are  indistinguishable  to  the  eye,  and  being 
coloured  with  the  three  primary  screen  colours,  so  that  the 
plate  as  a whole  appears  to  be  gray.  Such  a plate  is  put  with 
the  elements  in  contadl  with  the  film  of  a panchromatic  plate, 
which  has  been  so  adjusted  to  the  screen,  either  by  means  of 
modification  in  the  making  of  a plate  or  by  a compensating 
filter,  that  white  light  produces  equal  effect  through  each  or 
the  three  colour  elements. 

If  a negative  of  a coloured  object  be  taken  on  the  plate 
through  such  a screen,  and  a positive  be  made  from  the  nega- 
tive, this  positive  being  registered  again  upon  the  same  screen, 
we  shall  obtain  a reprodudtion  of  the  coloured  object  by  addi- 
tive synthesis. 

Thus,  in  the  diagram,  fig.  46,  we  have  three  patches  of  red, 
green,  and  yellow  light  falling  upon  the  screen,  which  is  repre- 
sented only  by  three  unit  elements.  The  red  light  penetrates 
the  red  elements,  producing  blackness  in  the  negative  film 
beneath  them.  The  green  light  penetrates  the  green  ele- 
ments, producing  blackness  in  the  film  beneath  them.  The 
yellow  light,  composed,  of  course,  of  a mixture  of  red  light  and 
green  light,  penetrates  both  these  elements,  leaving  the  nega- 
tive film  transparent  only  under  the  blue  elements.  When  the 
positive  is  registered  again  upon  the  screen,  the  red  light  is 

86 


THREE-COLOUR  PHOTOGRAPHY 


represented  by  the  clear  red  elements,  the  green  and  blue  ones 
being  obstructed.  In  the  same  way  the  green  is  represented 
by  clear  green  elements,  the  red  and  blue  being  obstructed, 
and  the  yellow  is  represented  by  the  light  coming  from  the 
mixed  red  and  green  elements,  the  blue  only  being  obstructed. 
It  will  be  seen  that  if  the  negative  instead  of  the  positive  be 
registered  upon  the  screen,  the  complementaries  to  the  original 
colours  will  be  obtained;  the  red  will  be  represented  by  a mix- 
ture of  green  and  blue,  the  green  by  the  magenta  colour  re- 
sulting by  mixing  the  red  and  blue,  and  the  yellow  by  blue. 

Professor  Joly,  who  used  exaCtly  the  method  described,  made 


NEGATIVE 


PROJECTING 
SCREEN 
FILM  OF  POSI- 
TIVE 


RED  QRELN  YELLOW! 


Fig.  46 


his  taking  screen  of  wide-banded  colours  somewhat  resembling 
the  colour  mixture  curves,  while  the  viewing  screen  on  which 
the  positive  was  registered  had  deep  narrow-banded  colours. 

The  Lumiere  Autochrome  plate,  on  the  other  hand,  is  not 
worked  in  quite  the  same  way.  The  colour  elements  in  this 
are  very  small,  being  composed  of  flattened  starch  grains,  and 
the  emulsion  is  coated  on  the  plates.  After  exposure  and 
development  the  image  in  the  emulsion  is  reversed  so  that  it 
is  converted  into  a positive  and  the  colours  can  be  seen  at  once. 

This  necessitates  the  use  of  the  same  filters  for  taking  and 
viewing,  these  filters  dividing  the  speCtrum  almost  exaCtly  into 
three  equal  parts.  The  exactness  of  colour-representation  of 

87 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


the  Lumiere  Plate  would  seem  to  justify  such  a procedure, 
though  undoubtedly  pure  deep  reds  would  be  better  rendered, 
if  a less  orange-red  could  be  used  for  the  viewing  filter. 

There  are  many  systems  of  screen-plate  photography,  all 
similar  in  principle  either  to  the  Lumiere  Autochrome  or  the 
Joly  method;  for  a general  theoretical  discussion  of  the  subject 
the  leCture  of  Dr.  Mees  to  the  Society  of  Arts,  published  in  the 
u Journal  of  the  Society  of  Arts,”  vol.  lvi,  p.  195,  and  a lengthy 
paper  published  in  “ The  Photographic  Journal  ” for  May  19 10, 
may  be  consulted.  For  pradical  instruction  there  are  one  or 
two  excellent  handbooks  and  numberless  articles  dealing  with 
the  matter. 


(2)  The  Subtraflive  Process 

The  Additive  methods  of  Synthesis  give  results  of  great 
accuracy  and  are  very  easy  to  work,  but  there  are  several 
defedts  connected  with  them.  Probably  the  greatest  is  the 
difficulty  of  obtaining  bright  pictures. 

The  triple  lantern  is  wasteful  of  light,  and  is  also  a very 
expensive  piece  of  apparatus.  Screen  plates  can,  at  best,  only 
give  one-third  of  the  brightness  which  should  be  given  by  their 
whole  surface,  and  they  therefore  require  powerful  light  sources 
to  show  them  satisfactorily.  Another  defeCt  is  that  the  addi- 
tive processes  cannot  give  paper  prints,  at  any  rate  as  yet. 
For  these  reasons  the  subtraCtive  processes  are  of  more  practical 
importance  than  the  additive,  and  though  the  introduction  of 
commercial  screen-plates  has  greatly  increased  the  use  of  addi- 
tive methods,  the  faCt  that  the  subtraCtive  process  is  used  in 
commercial  three-colour  half-tone  work  makes  it  much  the  most 
widely  used  method. 

In  subtraCtive  processes  the  three  negatives,  through  the  red, 
green,  and  blue  filters,  are  taken  as  in  the  additive  process,  but 
they  are  printed,  not  as  transparencies  to  be  projected  by  coloured 
light,  but  as  three  superposed  prints,  each  print  being  made  in  a 
colour  which  is  complementary  to  that  of  the  taking  filter. 

Thus,  if  we  divide  the  speCtrum  so  that  we  consider  white 
light  to  be  made  up  of  red,  green,  and  blue,  then  the  negative 
taken  hy  red  light  is  printed  in  a colour  which  transmits  or 
refleCts  all  the  green  and  all  the  blue,  simply  absorbing  the  red. 
In  the  same  way  the  negative  taken  by  green  light  is  printed 
in  a magenta  colour,  which  transmits  all  the  red  and  all  the 
blue,  absorbing  the  green.  The  negative  taken  by  blue  light 
is  printed  in  yellow,  which  transmits  all  the  red  and  all  the 

88 


Print  from  Blue  Filter  Negative 


Print  from  Green  Filter  Negative 


Print  from  Red  Filter  Negative 


THREE-COLOUR  PHOTOGRAPHY 


green,  but  absorbs  the  blue.  Let  us  turn  now  to  the  fig.  47, 
which  shows  six  patches  of  colour  consisting  in  the  top  line  of 
red,  green,  and  blue,  and  in  the  bottom  of  their  comple- 
mentaries,  blue-green,  magenta,  and  yellow.  In  the  red  negative 
we  shall  record  as  black  the  red  patch,  and  also  the  magenta 
and  yellow  patches,  by  virtue  of  the  red  light  reflected  from 
them.  If  we  print  this  in  a blue-green  ink  we  shall  print  blue- 
green  wherever  there  was  no  red  in  the  original;  that  is  to  say, 
in  the  position  of  the  green  patch,  the  pure  blue  patch,  and  the 
blue-green  or  minus  red  patch. 

The  green  negative  will  record  as  black  the  green  patch,  and 
the  blue-green  and  yellow  patches  by  virtue  of  the  green  light 
reflected  from  them.  Printing  it  in  magenta  we  shall  print 
magenta  ink  in  the  positions  of  the  red  patch,  the  pure  blue 
patch,  and  the  magenta  patch,  these  being  the  patches  from 
which  no  green  light  was  reflected.  In  the  same  way  the  blue 
filter  negative  will  record  as  black  those  patches  from  which  blue 
light  is  reflected;  that  is,  the  blue  patches,  and  the  magenta 
and  blue-green  patches.  This  is  printed  in  yellow  ink,  so  that 
we  print  yellow  in  the  places  where  no  blue  is  reflected ; that 
is,  in  the  positions  of  the  red  patch,  the  green  patch,  and  the 
yellow  patch.  Superposing  these  three  printings,  as  is  done  in 
the  original  figure,  we  obtain  red  by  the  printing  of  magenta 
ink  upon  yellow  ink,  green  by  the  printing  of  the  blue-green  ink 
on  the  yellow  ink,  and  blue  bv  the  printing  of  the  blue-green 
ink  upon  the  magenta  ink,  the  other  three  patches  being  pro- 
duced by  the  printing  of  the  three  inks  separately.  If  we  print 
all  three  inks  on  the  top  of  one  another  we  shall  get  a black,  or 
if  they  are  only  partially  printed  a scale  of  grays.  Of  the 
various  methods  for  actually  preparing  photographic  lantern 
slides  or  prints  in  the  three-colour  process  it  is  not  intended  to 
speak  in  this  book;  for  that  purpose  reference  should  be  made 
to  a book  on  three-colour  photography,  such  as  Dr.  Konig’s 
book  on  “ Natural  Colour  Photography,”  translated  by  Mr. 
E.  J.  Wall,  but  it  is  necessary  to  discuss  the  filters,  plates,  and 
printing  colours. 

The  question  as  to  the  filters  to  be  used  in  the  subtradfive 
process,  with  especial  reference  to  half-tone  work,  was  investi- 
gated by  Mr.  A.  J.  Newton  and  Mr.  A.  J.  Hull  (“  Photographic 
Journal,”  October  1904). 

They  came  to  the  following  conclusions: 

I.  It  is  not  possible,  nor  is  it  desirable,  for  any  filter  and 
plate  to  follow  either  the  colour  sensation,  colour  mixture,  or 
certain  other  calculated  curves. 

89 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

2.  The  effect  of  using  plates  having  maxima,  with  broad- 
banded  weak  filters,  is  to  cause  a degradation  of  any  pure  colour 
occurring  in  the  band  of  insensibility;  therefore  plates  showing 
gaps  in  the  spectrum  record  (erythrosine  plates,  e.g.)  should  not 
be  used  for  the  green  negative. 

3.  Ultra-violet  should  not  be  recorded,  as  it  will  exercise  a 
disturbing  effedt  where  it  is  recorded  by  colours  other  than  blues 
and  violets,  as  is  the  case  with  some  browns,  scarlets,  and 
yellows,  these  reproducing  with  a distindt  bluish  tint. 

4.  As  much  red  should  be  recorded  as  possible. 

5.  There  should  be  no  unrecorded  gaps  in  the  visible  spedtrum, 
for  while  these  may  not  be  important  for  certain  mixed  colours 
of  pale  tints,  they  are  fatal  to  corredt  rendering  of  colours,  the 
spedtra  of  which  do  not  extend  beyond  the  gap. 

6.  We  think  that  we  have  proved  that  the  filter  records 
should  be  even,  end  abruptly,  and  overlap  each  other  as 
follows:  the  blue-violet  and  the  green  should  overlap  from 
4,600  as  far  as  5,000,  and  the  red  and  green  should  overlap 
from  5,800  to  6,000. 

These  recommendations  as  to  the  filter  transmissions  appear 
to  be  sound,  and  form  a sort  of  mean  to  the  pradlice  of  various 
workers.  The  crux  of  the  whole  matter  lies  in  the  question  of 
the  green  filter.  According  to  the  theory  of  the  subtradtive 
process,  pure  blues  are  produced  by  the  printing  of  magenta 
on  blue-green,  the  absorption  of  the  green  leaving  blue.  Un- 
fortunately most  magenta  inks,  and  indeed  most  dyes,  absorb 
far  too  much  blue,  and  when  printed  on  blue-green  they  leave 
only  a very  dark  and  violet-blue. 

In  order  to  avoid  this  it  is  desirable  to  prevent  full-strength 
magenta  being  printed  on  the  blue-green  by  recording  the  blue 
to  some  extent  in  the  green  filter  negative.  Unfortunately  the 
extension  of  the  green  filter,  in  order  to  allow  this,  involves  a 
difficulty  with  green,  and  especially  with  dark  and  yellow 
greens. 

The  exposure  which  is  given  to  the  green  filter  is  regulated, 
not  by  the  light  refledted  from  greens , but  by  the  light  refledted 
from  white  objedts. 

Now,  as  has  been  shown  in  Chapter  I,  green  objedts  show  a 
considerable  absorption  of  the  green  light  itself,  and  conse- 
quently if  the  green  filter  is  broader  than  their  region  of  strong 
refledtion,  green  objedts  will  appear  very  dark  and  be  much 
under-exposed.  The  result  of  this  will  be  that  some  magenta 
will  be  printed  on  the  greens.  Moreover,  the  blue-green  ink  or 
dye  very  rarely  contains  sufficient  green,  so  that  greens  are 

90 


THREE-COLOUR  PHOTOGRAPHY 


usually  too  dark,  and  the  effect  of  the  printing  of  a small  quan- 
tity of  magenta  is  very  serious. 

It  is  necessary  to  compromise,  and  probably  the  best  effect  is 
got  by  a green  filter  which  is  somewhat  narrower  than  that 
suggested  by  Newton  and  Bull.  The  Wratten  tricolour  green 
filter  extends  from  6,000  to  4,800.  With  modern  red-sensitive 
plates  it  is  necessary  that  the  green  filter  should  not  have  any 
appreciable  extension  into  the  red.  Red  colours  are  usually 
very  luminous,  absorbing  little  red  light,  and  if  they  are 
transmitted  by  the  green  filters  they  will  record,  preventing  the 
printing  of  sufficient  magenta,  and  making  the  resulting  colour 
too  orange. 

This  great  luminosity  of  red  colours  is  probably  the  explana- 
tion of  that  somewhat  puzzling  phenomenon,  the  success  of  the 
three-colour  process  before  the  use  of  plates  really  sensitive  to 
red  light. 

It  was  shown  by  Dr.  S.  E.  Sheppard  and  Dr.  Mees  (“Phot. 
Journ.,”  March  1906)  that  more  than  half  the  record  through 
many  red  filters  upon  the  plates  then  in  use  was  made  by  waves 
of  less  length  than  6,000  A.U.  But  pure  reds  are  so  luminous, 
even  near  their  absorption  band,  that  they  are  capable  of  record- 
ing sufficiently  by  such  rays  alone. 

•It  has  often  been  suggested  that  the  blue  filter  should  transmit 
some  of  the  extreme  red.  Many  such  filters  are  in  existence,  and 
it  has  been  said  that  the  use  of  such  filters  represents  a departure 
of  sound  practice  from  “ theory.”  The  cause  of  the  existence 
of  such  filters  is,  undoubtedly,  that  it  is  difficult  to  make  a 
bright  blue  filter  in  which  the  red  is  completely  absorbed. 

But  as  to  the  “ sound  practice  ” it  must  be  explained  that 
until  pinacyanol  came  into  use  there  were  no  plates  which 
recorded  the  extreme  red  transmitted  by  these  filters,  and  con- 
sequently it  was  simply  of  no  importance. 

Even  a filter  made  of  methyl-violet,  which  appears  quite 
purple  by  daylight  and  bright  red  by  gaslight,  gives  no  red 
record  at  all  on  a pinachrome  plate,  while  on  a pinacyanol  bathed 
plate  the  effedt  of  the  spedtrum  of  daylight  is  about  nine  times 
as  great  at  the  blue  end  as  at  the  red. 

If,  however,  a filter  such  as  rhodamine,  which  transmits  all 
the  red  and  orange,  be  used  with  a Wratten  panchromatic  plate, 
the  effedt  will  be  that  a scarlet  colour  will  register  in  the  blue- 
filter  negative  as  well  as  in  the  red,  and  consequently  will  be 
represented  by  magenta  printed  alone.  With  such  a filter  bright 
yellow  will  record  in  all  three  negatives,  and  be  left  as  white  in 
the  print. 


9 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

# 

For  these  reasons  there  is  no  doubt  that  the  blue  filter  should 
not  record  any  red  whatever. 

It  is  of  great  importance  in  three-colour  work  that  the  three 
negatives  should  be  of  the  same  gradation  as  nearly  as  possible. 
Should  this  not  be  so,  a scale  of  grays  produced  by  the  super- 
position of  three  printings  will  differ  in  colour  at  the  two  ends. 
Such  a scale  may,  for  instance,  have  the  lighter  tones  of  a bluish 
tint,  while  the  deep  tones  are  brownish.  In  order  that  the  three 
negatives  should  be  identical  in  steepness  of  gradation,  it  is 
necessary  that  they  should  all  three  be  made  on  exadtly  the 
same  kind  of  plate.  Plates  vary  very  greatly  in  the  rate  at 
which  they  develop,  this  rate  varying  in  two  consecutive 
batches  of  plates  made  in  the  same  way.  Consequently,  if  the 
three  negatives  be  made  on  three  different  kinds  of  plate,  even 
if  these  be  simply  the  same  plate  bathed  in  different  dyes,  the 
rates  of  development  are  unlikely  to  be  the  same,  and  the  grada- 
tions will  be  different  to  some  extent. 

It  was  formerly  customary  to  use  three  kinds  of  plate  sensitized 
for  the  special  spedlral  regions  transmitted  by  each  filter.  This 
was  necessary  because  only  in  this  way  could  the  shortest 
possible  exposure  under  each  filter  be  obtained.  With  the  in- 
troduction of  the  satisfactory  W ratten  panchromatic  plates 
■sensitive  to  the  whole  speCtrum,  this  reason  has  disappeared, 
and  it  is  now  certainly  the  best  practice  to  use  the  same  pan- 
chromatic plate  for  all  three  exposures.  Such  a panchromatic 
plate,  however,  necessitates  the  employment  of  filters  which  cut 
accurately  at  the  required  points.  As  pointed  out  above,  a red 
transmission  band  through  the  blue  filter  is  of  no  importance  if 
the  plate  employed  be  not  sensitive  to  red,  but  if  a panchromatic 
plate  be  used,  the  blue  filter  must  rigidly  cut  out  red  light. 

Mr.  Chapman  Jones  and  Sir  William  Abney  have  shown  that 
light  of  various  colours  does  not  produce  the  same  gradation 
upon  photographic  plates.  Experiments  by  E.  Stenger  show 
that  the  same  statement  is  to  a less  extent  true  of  plates  sensit- 
ized with  the  isocyanine  dyes. 

A large  number  of  measurements  of  the  W ratten  panchromatic 
plate,  when  exposed  through  the  three  filters  used  in  tricolour 
photography,  have  been  made,  and  they  show  that  the  variation 
in  gradation  between  the  three  curves  is  inappreciable,  being 
very  much  less  than  in  erythrosine  plates,  or  even  plates  sensit- 
ized with  any  single  dye.  Consequently,  it  may  be  safely 
assumed  that  if  the  three  negatives  are  made  on  the  same  plate 
or  on  three  plates  from  the  same  box,  and  are  developed  to- 
gether, the  gradation  of  the  three  negatives  will  be  identical. 

92 


THREE-COLOUR  PHOTOGRAPHY 


In  order  to  help  in  this  matter,  the  W ratten  copyboard  chart 
was  put  on  the  market.  This  contains  a neutral  gray  graded 
strip  going  by  steps  from  black  to  white.  I'his  strip  should  be 
rendered  by  equal  density  in  all  three  negatives  (and  will  be)  if 
the  exposure  is  correct,  and  the  development  the  same.  The 
chart  also  contains  marks  to  make  register  easy,  and  three  colour 
patches  which  enable  the  respective  negatives  to  be  immediately 
identified. 

The  real  difficulty  of  the  subtraCtive  processes  lies  in  the 
selection  of  the  printing  colours. 

These  printing  colours,  by  whatever  method  they  are  to  be 
applied,  whether  as  stained  gelatine,  in  the  stripping  film  or 
other  carbon  processes,  as  dyes,  in  the  pinatype  process,  or  as 
printing  inks,  must  be,  as  nearly  as  possible,  complementary  to 
the  taking  filters.  The  reason  being  that  we  start  with  the 
assumption  that  the  paper  on  which  we  print  (or  the  viewing 
surface,  if  transparencies  are  in  question)  is  reflecting  blue,  green, 
and  red,  comprising  all  colours,  which,  when  blended  together, 
look  white.  Now,  as  already  stated  above,  if  we  print  from  the 
red  record  negative,  we  are  printing  from  the  shadow  portions 
of  the  negative,  that  is  where  there  was  an  absence  of  red  in 
the  original  and  therefore  no  record,  so  that  we  must  print  in  a 
colour  that  completely  absorbs  all  red.  At  the  same  time,  there 
may  have  been  some  other  colour  in  this  portion  of  the  original, 
so  that  not  only  must  the  red  be  absorbed,  but  the  green  and 
blue  must  be  completely  reflected.  Therefore  this  colour  will 
be  a minus-red  (blue-green).  In  the  same  way  the  material  in 
which  the  shadows  of  the  green  record  negative  should  be 
printed  will  be  a minus-green  (magenta),  which  entirely  absorbs 
the  green  and  refleCfs  all  the  blue  and  red,  and  the  blue  record 
negative  must  be  printed  in  a minus-blue  (yellow),  which  entirely 
absorbs  the  blue,  and,  at  the  same  time,  completely  reflects  the 
red  and  green. 

Although  it  is  not  yet  possible,  and  perhaps  never  will  be,  to 
obtain  perfeCt  printing  colours,  we  know  what  is  the  ideal,  and 
we  can  therefore  see  what  defeCts  will  usually  occur.  Shortly 
stated,  the  main  trouble  is  that  they  do  not  sufficiently  absorb 
the  colours  they  should  absorb,  and  do  not  refleCt  sufficiently 
the  colours  they  should  refleCE  Although  yellow  is  generally 
the  most  satisfactory  of  the  three,  it  reflects  nearly  io  per  cent, 
of  the  blue  and  violet  it  should  completely  absorb,  and  it  absorbs 
about  the  same  amount  of  the  colours  it  should  refleCt  entirely. 
The  red  (magenta)  printing  colour  which  should  absorb  all  the 
green  and  refleCt  all  the  blue  and  red,  does  not  absorb  completely 

93 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

the  wave-lengths  represented  by  the  filter  records;  in  parts 
there  is  a refledtion  of  20  per  cent.,  and  even  in  the  colours 
that  are  best  absorbed  there  is  still  some  reflection.  The  colours 
that  should  be  entirely  reflected  are  worse,  since  from  25  per 
cent,  to  60  per  cent,  of  each  of  these  colours  is  absorbed,  instead 
of  being  reflected,  blues  and  violets  being  very  imperfectly  re- 
flected. The  blue  printing  colour  is  worse  still;  it  absorbs  from 
50  per  cent,  to  60  per  cent,  of  its  own  brightest  colour,  nor 
does  it  absorb  the  red  completely. 

Now  we  can  see  from  this  that  we  must  expeCt  to  find 
defeCts  in  the  colour  rendering,  even  when  the  filters  and  plates 
are  perfeCt,  and  exposure,  development,  and  printing  have  been 
quite  correCt,  and  further  consideration  will  show  what  sort  of 
errors  to  expeCt. 

A red  must  be  reproduced  by  full  printing  strength  yellow 
and  full  strength  magenta.  Now  the  yellow  allows  some  of  the 
blue  to  be  reflected,  and  the  magenta  allows  so  much  of  the 
yellow  and  yellow-green  to  be  reflected,  that  the  result  will  be 
a grayish  orange  rather  than  a true  red. 

Yellow  will  be  reproduced  by  yellow  alone.  This  will  be 
fairly  satisfactory,  except  that  there  is  some  degradation  due  to 
the  absorption  of  red  and  yellow  and  the  reflection  of  some 
blue,  these  defeCts  making  the  colour  somewhat  grayer  than  it 
should  be. 

Green  will  be  reproduced  by  printing  full  strength  yellow 
and  full  strength  blue.  But  some  of  the  green  is  absorbed  by 
the  yellow  ink  and  a much  larger  amount  by  the  blue  ink, 
therefore  the  green  will  look  a dark  yellowish-green,  rather 
than  a bright  pure  green. 

A blue-green  will  be  matched  by  the  printing  colour  itself, 
but  this  will  be  darker  than  it  ought  to  be,  because  it  absorbs 
over  half  of  its  own  colour  instead  of  reflecting  all. 

A pure  blue  will  be  reproduced  by  full  strength  blue  and  full 
strength  magenta,  but  this  is  degraded,  because  neither  of  the 
pigments  refleCt  all  the  blue  as  they  should. 

Black  will  not  be  a dense  neutral  black,  but  either  greenish 
black  or  reddish  black,  according  to  whether  the  red  has  in- 
sufficiently absorbed  the  green,  or  the  blue  has  insufficiently 
absorbed  the  red. 

Because  certain  colours  do  not  reproduce  so  well  as  desired, 
it  is  customary  to  blame  either  the  plates,  or  filters,  or  both,  or 
to  say  that  something  is  wrong  with  the  theory  underlying  the 
process.  But  this  is  not  the  case;  it  is  a sufficient  proof  that 
the  theory  is  correct  and  that  the  plates  and  filters  are  suitable, 

94 


THREE-COLOUR  PHOTOGRAPHY 


to  cite  the  results  obtained  by  an  additive  process  (as  Kromscop 
or  screen  plate  processes)  which  gives  results  that  are  marvel- 
lous in  their  approximation  to  the  truth. 

Attention  is  often  called  to  greens,  and  it  is  pointed  out 
that  greens  seldom  have  a dense  deposit  on  the  negative  taken 
under  the  green  filter,  and  therefore  the  plate  is  insufficiently 
green  sensitive.  This,  however,  is  not  the  case,  the  plate 
being  perfectly  sensitive  to  green  light,  but  the  reason  is  that 
only  a small  proportion  of  pure  green  light  is  reflected  from 
any  green  objeCt.  Thus,  measurements  made  bv  A.  J.  Bull 
proved  that  in  the  case  of  fir,  holly,  and  ivy,  the  maximum 
amount  of  green  reflected  was  only  8 per  cent.,  n per  cent., 
and  12  per  cent,  respectively,  of  the  amount  falling  upon  them, 
while  even  young  beech  leaves,  which  are  so  bright  in  the 
spring,  refledt  only  32  per  cent.  Two  greens  taken  from 
water-colour  sketches  reflected  only  30  per  cent,  and  48  per 
cent.,  and  a very  lightly  printed  emerald  green  printing  ink  re- 
flected only  72  per  cent,  of  green,  while  at  the  same  time  re- 
flecting a considerable  amount  of  blue  and  red.  Mr.  Bull  states 
therefore  that  “ it  is  quite  proper  that  common  greens  should 
not  be  recorded  by  full  density  on  the  photographic  plate. 

“ If  we  now  consider  in  what  manner  a green  is  reproduced  by 
the  three-colour  process,  we  find  that  a green  is  only  photo- 
graphically recorded  in  the  green  negative  (red  printing  plate), 
the  other  two  negatives  treating  it  as  a black.  The  green  is 
therefore  primarily  rendered  by  the  superposition  of  the  printing 
inks  used  for  the  red  and  blue  negatives,  namely,  blue  ink  and 
yellow  ink,  and  in  addition  there  is  printed  an  amount  of  red,  or 
rather  magenta,  ink,  which  is  smaller  in  proportion  as  the  green 
subjeCt  has  been  photographed  in  the  green  negative.  But,  as 
we  have  seen,  the  amount  of  green  light  reflected  by  green 
objeCts  rarely  approaches  the  amount  of  green  light  reflected  by 
a white,  and  it  therefore  follows  that  there  must  always  be  some 
considerable  amount  of  magenta  ink  printing  on  the  greens  in 
order  to  render  them  accurately  were  the  process  correCt. 

“This  is  an  inherent  part  of  the  process  and  would  present 
no  difficulty  if  the  superposed  blue  and  yellow  inks  produced  a 
green  which  reflected  praCtically  all  the  green  light  which  fell 
upon  it,  but  this,  unfortunately,  is  not  the  case ; the  combina- 
tion of  the  available  yellow  and  blue  inks  produces  a some- 
what dark  green,  and  the  crimson  degrades  it  still  more.  This 
is  the  chief  cause  which  tends  to  mar  the  mechanical  reproduc- 
tion of  greens.” 

Another  defeCt  of  which  complaint  is  made  is  that  there  is 

95 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


too  much  yellow  in  the  violets  and  that  therefore  the  blue 
filter  must  be  wrong.  This,  however,  is  also  erroneous,  and 
the  fault  is  again  due  to  the  inks.  Some  yellow  would  be 
necessary  if  the  inks  were  corredl,  otherwise  the  blue-green  and 
magenta  inks  would  give  only  some  shades  of  pure  blue.  In  any 
case  it  is  not  possible  to  make  a filter  that  will  transmit  more 
violet,  as  will  easily  be  seen  if  a photograph  is  made  with  the 
blue  filter  and  without  any  filter  at  all.  The  amount  of  violet 
recorded  will  be  as  great  as  it  is  even  with  no  filter,  whereas 
the  pale  greens  and  yellows  which  are  stopped  by  the  filter  will 
be  recorded  when  none  is  used. 

The  pradlical  importance  of  the  colours  of  the  inks  used  for 
tricolour  printing  has  caused  us  to  offer  two  devices  to  aid  in 
the  selection  of  these  inks,  namely,  the  Wratten  Ink  Tester  for 
the  visual  examination  of  the  colours  of  commercial  printing 
inks,  and  also  the  Wratten  Ink  Control,  a set  of  tricolour 
blocks  which  enables  engravers  and  printers  to  make  a definite 
practical  test. 


96 


CHAPTER  XI 


THE  OPTICAL  PROPERTIES  OF  FILTERS 

IT  is  curious  to  note  that  in  books  dealing  with  photographic 
optics,  and  indeed  in  practically  all  photographic  literature, 
the  optical  properties  of  filters  are  ignored.  In  spite  of  the  im- 
mense amount  of  attention  which  has  been  devoted  to  attaining 
the  wonderful  definition  given  by  the  modern  anastigmatic 
lens,  we  know  of  no  mention  of  the  fairly  obvious  fadt  that  the 
definition  of  any  lens  can  be  completely  ruined  by  a bad  filter, 
and  that  a considerable  number  of  filters  do  actually  degrade 
the  definition  of  the  lenses  with  which  they  are  used. 

It  is  frequently  stated  in  photographic  periodicals  that  a 
simple  and  cheap  way  of  preparing  a filter  is  to  dye  a fixed 
out  lantern  plate  and,  after  drying,  use  it  as  a filter  on  a portrait 
lens.  While  this  process  is  simple  and  cheap  the  loss  of  defini- 
tion is  so  serious  that  probably  few  careful  workers  would  be 
satisfied  with  such  a makeshift. 

In  order  to  investigate  the  optical  properties  of  filters  Messrs. 
Wratten  and  Wainwright,  Ltd.,  ereCted  a large  testing  instru- 
ment, consisting  of  a lens  of  great  focal  length  forming  an  image 
of  a distant  objeCf,  which  image  can  be  examined  by  means 
of  an  eye-piece  sliding  upon  an  optical  bench.  The  filter  to  be 
tested  is  placed  upon  the  front  of  the  lens  and  the  aberrations 
which  it  introduces  in  the  image  are  determined.  The  aberra- 
tions introduced  by  a filter  will  vary  as  the  square  of  the  focal 
length  of  the  lens,  so  that  as  the  focal  length  of  the  lens  is 
about  five  feet,  and  the  magnifying  power  of  the  eye-piece  is 
fourteen,  the  linear  size  of  the  aberrations  is  1,400  times  as 
great  as  those  which  the  filter  would  produce  upon  a lens  of 
six  inch  focus.  An  improved  instrument  of  this  type  has  been 
installed  by  the  Eastman  Kodak  Company  at  Rochester. 

The  errors  introduced  by  any  filter  can  thus  be  rigidly  deter- 
mined, and,  provided  that  the  lens  with  which  it  is  to  be  used 
is  known,  it  is  possible  to  ensure  that  the  filter  is  quite  suitable 
for  its  purpose. 


97 


G 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

Errors  in  filters  tending  to  produce  aberrations  and  imperfect 
definition  are  of  two  kinds;  those  inherent  in  the  glass,  and 
those  produced  by  strains  induced  in  manufacture  or  mounting. 

The  first  class  of  errors  can  only  be  avoided  by  careful 
selection  of  the  glass,  though,  unfortunately,  in  this,  as  in  all 
other  optical  work,  accuracy  involves  expense.  Patent  plate  is 
not  good  enough  in  surface  for  the  preparation  of  filters,  and 
even  filters  for  small  lenses  require  to  be  made  from  glass  of 
considerably  higher  quality.  If  filters  of  the  highest  possible 
accuracy  are  to  be  obtained,  then  the  glasses  into  which  they 
are  cemented  should  be  optically  surfaced  and  tested  in  the  same 
way  as  lenses.  When  filters  are  intended  for  microscopic  work, 
or  for  other  purposes  where  they  are  not  to  be  used  upon  a lens, 
as,  for  instance,  in  speCtroscopy,  it  is  not  necessary  for  the  figure 


In  “ B ’ Glass  In  Glass  (Flats) 

Fig.  48.  Cemented  “ K ” Filters 

of  the  glass  to  be  perfeCt,  provided  that  the  glass  is  free  from 
surface  imperfections,  reasonably  flat,  as  flat  as  a good  patent 
plate,  for  instance,  and  preferably  white  optical  glass,  not  green 
glass. 

The  Eastman  Kodak  Company  prepare  filters  in  glass  of 
three  qualities : 

A.  Optical  Flats,  accurately  surfaced  by  the  finest  optical 

glass  workers  in  the  world. 

B.  Picked  optical  glass  for  use  on  lenses,  and 

C.  White  optical  glass  of  less  perfeCt  figure,  this  glass 

being  only  used  for  special  filters,  such  as  microscopic 
or  speCtroscopic  filters,  which  are  used  in  such  a 
position  that  they  cannot  introduce  aberrations. 

The  second  class  of  error  is  produced  by  strains  to  which  the 
glass  is  subjected.  Many  filters  are  made  by  coating  dyed  gela- 

98 


OPTICAL  PROPERTIES  OE  FILTERS 


tine  upon  glass,  but  this  method  has  the  disadvantage  that  the 
drying  of  the  gelatine  bends  the  glass  into  a saucer  shape.  If 
then  a second  coated  glass  is  cemented  to  the  first  by  Canada 
balsam,  a lens  is  produced,  which  may  seriously  alter  the  focus 
of  the  lens  (this  is  shown  with  the  effect  exaggerated  in  fig.  49), 
while  any  greater  bending  in  one  direction  than  in  the  other 
will  destroy  the  definition  by  introducing  astigmatism.  In  order 
to  avoid  this  defeat  filters  may  be  made  by  coating  dyed  gelatine 
upon  prepared  plate  glass,  and  then,  after  drying,  stripping  the 
gelatine  from  the  glass.  These  gelatine  films  can  be  sold  at  low 
prices  as  “ film  filters,”  and  when  handled  with  care  they  are 
perfectly  satisfactory.  While  it  is  to  be  recom- 
mended that  for  permanent  use  cemented  filters 
should  be  employed,  because  the  film  filters  de- 
teriorate, yet  the  latter  are  convenient  for  experi- 
mental work,  and  give  results  equal  as  far  as  colour 
correction  goes  to  the  cemented  filters.  No  attempt 
should  be  made,  for  photographic  work,  to  proteCt 
the  films  by  binding  them  between  cover  glasses, 
because,  even  if  the  cover  glasses  themselves  are 
quite  free  from  any  tendency  to  introduce  aberra- 
tions, the  uncemented  films  thus  bound  up  intro- 
duce complex  reflection  effeCts  which  greatly 
affeCt  the  definition. 

A verv  important  point  in  the  preparation  of 
colour  filters  is  to  ensure  that  all  the  filters  of  a 
special  kind  which  are  prepared  are  accurately  of 
the  same  depth  of  colour.  The  Wratten  film 
filters  are  tested  in  the  following  manner:  after  j'jlters^ro- 
a quantity  of  film  has  been  coated  and  stripped,  duced  by 
specimens  are  seleCted  at  random  and  are  com- Coated  Glass 
pared  with  a standard  piece  of  film  upon  a very 
accurate  measuring  instrument  called  a speClrophotometer. 
This  instrument  indicates  whether  the  tint  of  the  batch  is  iden- 
tical with  that  of  the  standard,  and  whether  the  depth  of  the 
tint  is  also  normal.  Every  sheet  of  film  is  then  compared  visu- 
ally in  four  positions  with  a sheet  of  standard  film  in  a specially 
designed  comparator  instrument,  and  any  sheet  which  departs 
by  more  than  a given  amount  (usually  2 per  cent.)  from  the 
standard  is  rejected.  The  rejections  are  very  considerable,  be- 
cause even  with  the  greatest  care  variations  in  the  gelatine  or  in 
the  rate  of  drying  will  produce  alterations  in  the  colour  of  the 
resultant  filters,  and  often  batches  are  rejected  altogether,  while 
even  of  a first-rate  batch  many  sheets  will  depart  too  widely 

99 


Fig.  4.9. 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

from  the  standard ; but  the  method  ensures  that  all  the  films 
actually  passed  are  of  the  same  colour. 

After  cementing  the  films  into  selected  glass  by  means  of 
Canada  balsam  the  filters  are  dried  for  from  three  to  six  weeks 
according  to  size  at  a constant  temperature  in  a special  drying 
cupboard.  This  slow  drying  is  necessary  to  prevent  strains  in 
the  glass  being  produced  by  unequal  contraction  of  the  balsam. 
Such  strains  are  very  likely  to  occur  if  the  filters  are  dried  too 
quickly  or  at  unequal  temperatures. 

The  danger  of  strain  is  much  lessened  by  using  glass  of  con- 
siderable thickness,  and  for  this  reason  the  larger  filters  are 
usually  made  in  thick  glass,  while  filters  made  from  optical  flats 


( a ) Original  definition 

Fig.  50.  The  Effect  of  Straining  a Filter 


(b)  Definition  after  screwing  up 
tightly  in  cell 


are  no  less  than  half  an  inch  in  thickness,  so  that  no  danger  of 
strain  remains. 

After  drying  and  cleaning,  the  filters  are  carefully  tested  for 
freedom  from  aberration,  and  are  then  ready  for  mounting  in 
the  fitting  which  is  to  carry  them  for  use. 

Here  also  care  must  be  taken  that  pressure,  and  especially 
uneven  pressure,  is  not  put  upon  the  filter  in  mounting.  Filters 
are  frequently  fastened  into  cells  by  means  of  a screw  clamping 
ring,  and,  if  this  ring  has  not  a shoulder  upon  it,  it  is  sometimes 
screwed  tight  down  on  to  the  filter  in  order  to  hold  it  tightly. 
Such  a procedure  is  almost  certain  to  distort  a thin  filter  and 
spoil  the  definition,  as  is  shown  in  fig.  50,  where  (a)  shows  the 
original  definition  given  by  the  filter  and  (b)  that  obtained  after 
screwing  the  filter  up  in  its  cell. 

IOO 


OPTICAL  PROPERTIES  OF  FILTERS 


The  same  defeat  may  be  induced  by  binding  up  the  filters  at 
the  edge  with  strips  of  gum  paper  such  as  lantern  binding  strip. 
Even  a label  which  extends  over  both  glasses  of  a thin  filter  may 
cause  sufficient  distortion  to  make  the  filter  very  inferior. 

The  tests  through  which  filters  pass  for  optical  definition  are 
graduated  according  to  the  class  of  filter. 

The  Wratten  test  for  Class  A filters,  cemented  in  optical 
flats,  requires  that  the  finished  filter  shall  in  no  way  degrade  the 
definition  of  the  test  lens  (of  five  feet  focal  length)  when  used 
at  full  aperture.  Now  this  lens  is  specially  designed  to  give  the 
best  possible  definition,  and  the  test  object  for  flats  contains  a 
number  of  fine  lines  which  are  separated  from  each  other  only 
i 2,000  inch.  So  that  the  test  requires  that  a flat  filter  when 
used  on  a lens  of  sixty  inches  focal  length  at  full  aperture 
(about  inches)  shall  clearly  separate  lines  only  i 2,000  inch 
apart. 

This  test  may  seem  more  accurate  than  is  necessary,  but  by 
it  one  can  guarantee  flat  filters  to  give  perfect  definition  under 
any  circumstances,  even  in  high  power  telephoto  work,  or 
when  used  upon  large  lenses  covering  big  plates  for  severely 
critical  work. 

Ordinary  filters  cemented  in  optical  glass  of  good  quality,  but 
not  in  specially  surfaced  flats,  are  tested  in  a less  severe  manner. 
The  test  objedf  for  such  filters  is  shown  in  fig.  51.  This  test 
object  consists  of  a number  of  opaque  lines  crossing  each  other 
so  as  to  leave  transparent  squares.  In  the  instrument  the  image 
which  is  produced  has  lines  1 60  inch  across,  fig.  51  being  a 
photograph  of  the  image  enlarged  about  17  diameters. 

Fig.  52  shows  the  effect  on  the  image  produced  by  a filter 
showing  slight  astigmatism.  In  fig.  53  ( a ) and  (/>)  a filter  has 
been  used  in  which  the  astigmatism  is  very  severe. 

If  a filter  shows  astigmatism  the  set  of  black  lines  running  in 
one  direction  will  have  a focus  in  a different  plane  to  the  set  of 
lines  running  at  right  angles  to  them,  so  that  one  set  must 
always  be  out  of  focus  ; in  the  case  of  bad  astigmatism  it  is  pos- 
sible that  one  set  of  lines  will  disappear  entirely  when  the  other 
set  are  focussed  sharply,  so  that  in  Fig.  53  ( a ) shows  one  set 
of  lines  in  focus,  and  ( b ) those  at  right  angles  to  them. 

We  require  of  filters  in  B glass  that  at  full  aperture  the  image 
shall  be  free  from  astigmatism  and  that  the  definition  shall  be 
such  that  filters  up  to  two  inches  in  diameter  shall  not  visibly 
degrade  the  definition  of  a lens  of  six  inches  focus  used  at  F 4.5. 
Filters  between  two  inches  and  three  inches  in  diameter  must 
give  good  definition  on  a lens  of  ten  inches  focus  used  at  F 4.5, 

101 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


while  filters  above  three  inches  in  diameter  must  give  good  de- 
finition on  a lens  of  sixteen  inches  focus  used  at  F 8. 

For  very  long  focus,  or  telephoto,  lenses  only  flats  will  give 
satisfactory  results,  and  for  the  semi-telephoto  lenses,  now  being 


Fig.  51.  Filter-Test  Object  Fig.  52.  Test  Object  through 

Astigmatic  Filter 


introduced,  it  is  also  desirable  that  filters  should  be  cemented  in 
glass  of  the  highest  quality. 

On  stopping  down  the  definition  given  by  the  filter  should 
improve  as  the  aperture  is  diminished. 

In  addition  to  giving  satisfactory  definition  it  is  necessarv 


(*)  (6) 

Fig.  53 

that  a filter  should  not  alter  the  focus  when  it  is  used  upon  a 
lens.  A camera  should  always  be  focussed  with  the  filter  in 
position,  but  the  use  of  focussing  scales  upon  many  small  cameras 
renders  it  necessary  that  filters  for  hand  camera  use  should  not 
affeCt  the  focal  plane  of  the  lens.  *A  filter  can  alter  the  distance 

102 


1 


OPTICAL  PROPERTIES  OF  FILTERS 


between  the  lens  and  its  local  plane  in  two  ways,  it  may  ad  as 
a weak  supplementary  lens,  or  it  may,  if  behind  the  lens,  pro- 
duce an  effed  due  to  its  thickness. 

A u K ” filter  is  tested  not  to  affed  the  focal  plane  of  a six 
inch  lens  by  a greater  amount  than  i 50  inch  when  used  on  the 
front  of  the  lens.  If  a filter  is  used  behind  the  lens,  the  lens 
must  be  moved  back  by  an  amount  equal  to  about  one-third  of 
the  thickness  of  the  filter,  but  this  rule  assumes  that  the  filter 


Fig.  54.  Set  of  Tricolour  Filters  with  Fourth  Printer, 
CEMENTED  IN  FLATS 

does  not  ad  in  any  way  as  a lens,  and  it  is  probable  that  some 
filters,  which  have  been  noted  as  not  corresponding  to  the  rule, 
really  aded  to  some  extent  as  lenses. 

Besides  the  test  for  definition,  which  we  have  described,  a set 
of  filters,  such  as  tricolour  filters,  which  are  to  work  together, 
must  be  tested  for  another  optical  requirement.  'They  must 
give  images  of  the  same  size,  so  that  they  will  register  when 
printed  one  upon  another. 

It  is  obvious  that  if  the  filters  are  used  behind  the  lens,  or 

103 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

even  if  they  are  used  in  front  of  the  lens  and  the  object  be  near 
the  lens  (as  in  ordinary  picture  copying  or  process  work),  that 
the  filters  must  all  be  of  the  same  thickness,  and  that  the  shorter 
the  focal  length  of  the  lens  the  greater  the  error  in  register  (for 
the  same  size  of  image)  which  a difference  in  thickness  will 
introduce. 

For  some  time  it  was  thought  that  equal  thickness  was  the 
main  necessity  for  register,  but  when  the  construction  of  the 
large  Wratten  filter  testing  instrument  showed  that  filters  vary 
also  in  their  effeCt  upon  the  focal  length  of  the  lens,  it  appeared 
that  a complete  theoretical  and  practical  investigation  of  the 
optical  conditions  governing  register  was  desirable. 

In  order  to  measure  the  accuracy  of  registration,  an  instru- 
ment was  constructed  in  which  a lens  of  about  ten  inches  focal 
length  formed  images  eight  inches  apart  of  two  sharply  defined 
sets  of  lines,  and  the  exaCt  distance  between  these  sets  of  lines 
could  be  measured  by  means  of  two  microscopes  mounted  on 
carriages  aCtuated  by  micrometer  screws. 

Filters  can  be  placed  in  front  of  the  lens  and  the  effeCt  of 
these  filters  upon  the  size  of  the  images  can  then  be  measured 
very  accurately. 

The  tests  upon  this  instrument  have  completely  confirmed 
the  theoretical  sizes  calculated  from  the  known  laws  of  geo- 
metrical optics,  and  the  information  which  has  been  obtained  is 
of  considerable  use  in  ensuring  that  sets  of  tricolour  filters  will 
give  satisfactory  register,  while  tricolour  filters  cemented  in  flats 
(u  A ” glass  filters)  can  be  guaranteed  to  show  perfect  register 
under  even  the  most  trying  conditions. 

All  filters  issued  by  the  Eastman  Kodak  Company  are  tested 
on  the  two  instruments  described  for  definition,  accuracy  of 
focus,  and,  in  the  case  of  tricolour  filters,  for  register. 


CHAPTER  XII 


THE  FITTING  OF  FILTERS 

FILTERS  can  be  fitted  either  before  or  behind  the  lens,  or 
just  in  front  of  the  plate  in  a repeating  back  or  special  dark 
slide.  This  last  position  has  the  advantage  that  glass  of  lower 
optical  quality  can  be  used,  but  much  larger  filters  are  required 
and  any  speck  or  mark  upon  the  filter  shows  in  every  negative,  so 
that  for  orthochromatic  filters  a lens  fitting  is  certainly  to  be 
preferred.  Such  filters  should  preferably  be  fitted  in  front  of  the 
lens,  as  in  this  position  no  appreciable  shift  of  focus  is  introduced 


For  Flats  For  Filters  in  “ B”  Glass 

Fig.  55.  Screw  Cells 


by  the  thickness  of  the  glass,  and  the  filters  are  also  more  readily 
accessible. 

Film  filters  may  be  conveniently  placed  between  the  lens 
components,  but  this  cannot  be  done  with  cemented  filters, 
because  the  introduction  of  a piece  of  glass  would  seriously  affeCt 
the  corrections  of  many  lenses ; even  gelatine  film  should  not 
be  put  inside  a Cooke  lens,  which,  owing  to  its  construction,  is 
sensitive  to  small  alterations  in  the  air  spaces. 

The  neatest  method  of  fitting  filters  on  the  front  of  a lens 
is  the  mounting  of  the  filter  in  a screw  cell  (see  fig.  55),  which 
is  then  screwed  into  the  thread  cut  inside  the  front  lens  cell,  if 
the  lens  has  such  a screw  thread.  This  cell,  having  the  same 

105 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

diameter  as  the  lens  barrel,  enables  the  same  lens  cap  to  be  used, 
or  if  a roller  blind  shutter  or  lens  hood  is  used  it  will  fit  on  to 
the  screw  cell  in  the  same  way  as  on  to  the  lens. 

In  order  that  a filter  may  be  fitted  in  this  way,  either  the  lens 
or  the  front  combination  must  be  sent  when  ordering  such  a 
fitting,  as  the  cell  cannot  be  made  without  actual  fitting  to  the 
screw  thread. 

Screw  cells  which  are  made  for  filters  consisting  of  a single 
glass,  such  as  those  sometimes  issued  by  camera  makers,  contain- 
ing a brown  glass  filter,  cannot  be  used  for  holding  cemented 
filters,  but  must  be  replaced  by  a deeper  cell,  designed  to  hold 
the  thicker  filter.  They  a£t,  however,  as  admirable  patterns  for 
the  preparation  of  the  new  cell,  and  if  such  a cell  can  be  sent 
there  is  no  need  to  part  with  the  lens.  On  no  account  should 


For  Flats  For  Filters  in  “ B 5 Glass 

Fig.  56.  Slip-on  Cells 

cemented  filters  be  held  in  cells  by  burnishing  the  edge  of  the 
rim  over,  as  the  pressure  is  very  likely  to  strain  the  filter. 

A filter  cell  should  always  be  designed  so  that  the  filter  is 
held  securely  but  without  pressure,  and  if  the  filter  is  fastened 
in  place  by  a screw  ring  this  ring  should  have  a shoulder  and 
and  should  be  turned  down,  so  that  when  it  is  screwed  home 
the  filter  can  easily  be  turned  round  by  holding  it  between  the 
thumb  and  finger,  but  will  not  give  any  side  shake. 

Another  method  of  fitting,  which  is  rather  less  expensive,  is 
to  have  the  filter  mounted  in  a light  metal  cell,  which  is  slipped 
on  to  the  lens  like  a lens  cap  (see  fig.  56).  This  method  of 
fitting  has  the  advantage  that  the  filter  can  be  readily  removed 
or  changed. 

For  this  form  of  fitting  it  is  only  necessary  to  send  the  outside 
measurement  of  the  lens,  but  it  is  necessary  that  this  measure- 
ment should  be  made  very  exactly.  If  a pair  of  sliding  callipers 
cannot  be  obtained  a strip  of  hard  writing  paper  should  be 

106 


THE  FITTING  OF  FILTERS 


wrapped  round  the  lens  so  that  the  ends  overlap,  and  then  the 
two  pieces  of  paper,  where  they  just  overlap,  should  be  cut 
through  in  position  with  a sharp  knife  (see  fig.  57).  An  attempt 
to  cut  a strip  of  paper  which  will  just  go  round  the  lens  is  un- 
likely to  result  in  a measurement  sufficiently  accurate  to  ensure 
a well-fitting  cell. 

When  slip-011  metal  cells  are  used  there  is  some  danger  of 
pulling  the  cell  off  if  a lens  cap  be  used  over  it,  but  such  a cap 
is  unnecessary  if  a between-lens  shutter  be  used.  With  a Roller 
Blind  Shutter  it  is  better  to  fit  the  cell  on  the  back  of  the  lens, 


Fig.  57.  Cutting  a Slip  of  Paper  to  fit  a Lens 


unless  a screw  cell  can  be  used,  thus  leaving  the  front  free  for 
the  shutter. 

Filters  may  also  be  fitted  into  velvet  coloured  plugs  to  go 
inside  lens  hoods,  so  that  the  same  lens  cap  can  be  used,  or  a 
Roller  Blind  Shutter  will  fit  over  the  lens  without  difficulty. 
Roller  Blind  Shutters  can  be  adapted  to  take  filters  by  fastening 
wooden  or  metal  grooves  to  the  front  of  them,  and  using  square 
unmounted  filters,  or  a similar  fitting  can  be  slipped  on  to  the 
lens;  the  filters  can  be  easily  changed,  and  this  method  of  fitting 
will  be  found  quite  satisfactory  in  practice,  fig.  58. 

In  order  that  the  same  filter  may  be  used  on  different  lenses 
adjustable  holders  are  supplied.  These  are  made  to  contain 
circular  filters,  and  fit  on  lenses  having  diameters  from  } to  1 J 

107 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

inch,  £ to  if  inch,  and  f-  to  2f  inches.  Adjustable  holders 
are  also  made  for  square  filters  to  go  on  lenses  of  diameter  from 
-J  to  if  inch  and  from  if  to  2f  inches  (figs.  59,  60,  and  61). 

With  filters  cemented  in  flats  it  is  necessary  that  the  filter 
should  be  wider  than  the  front  lens  component,  because  other- 


Fig.  58.  Slip-on  Cell  to  take  Square  Filters 

wise  the  filter,  being  of  considerable  thickness,  wilPtend  to  cut 
down  the  angular  aperture  of  the  lens.  The  additional  width 
necessitated  can  be  found  approximately  in  the  following  manner 
(see  fig.  62):  On  a piece  of  paper  draw  a line  equal  to  the 
focal  length  of  the  lens,  at  right  angles  to  this  draw  a^line  equal 


Fig.  59.  Adjustable  Filter 
Holder  with  Square 
Filter 


Fig.  60.  Adjustable  Filter 
Holder  with  Circular 
Filter 


to  half  the  diagonal  of  the  plate  with  which  the  lens  is  used. 
Now  extend  the  first  line  for  a distance  equal  to  the  distance 
from  the  diaphragm  to  the  edge  of  the  hood  of  the  lens  plus 
three-quarters  of  an  inch,  and  at  right  angles  to  this  draw 
another  line.  If  now  we  join  our  starting  point  to  the  end  of 
the  line  representing  the  diagonal  of  the  plate,  and  produce  this 

108 


THE  FITTING  OF  FILTERS 


until  it  cuts  the  last  line  drawn,  the  length  of  that  line  which  it 
cuts  off  represents  half  the  necessary  width  of  the  filter. 


Fig.  6i.  Eastman  Adjustable  Filter  Holder 

Since  filters  in  flats  must  be  wider  than  the  lens  with  which 
they  are  to  be  used,  they  should  be  fitted  in  special  out-built 


Fig.  62 

cells,  which  preferably  screw  into  the  mount  of  the  front  lens 
component,  though  these  also  may  slip  on  to  the  lens  barrel  (see 
figs.^55  and  56). 

109 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 

In  order  that  there  should  be  no  mistake  when  ordering  filters 
in  “ A ” glass  (flats)  the  following  particulars  should  be  given  : 

1.  The  name  of  the  lens. 

2.  The  focal  length. 

3.  Maximum  working  aperture. 

4.  Length  and  diameter  of  lens  barrel,  over  all. 

5.  Size  of  plate  used. 

6.  If  used  for  other  than  ordinary  infinity  work  give  average 

extension  of  camera. 

7.  If  screw  cells  are  required  it  is  necessary  to  send  front 

combination,  and  if  slip-011  cells  are  required  it  is  better 
to  send  it  so  that  a good  fit  may  be  ensured  over  the 
hood. 


Fig.  63.  Tricolour  Slide-past  Fitting 

The  most  satisfaCtorv  way  of  fitting  a set  of  filters  in  flats  to 
two  or  more  lenses  is  to  have  screw  cells  to  fit  the  largest  lens 
and  then  to  obtain  adapters  which  can  be  screwed  on  to  the 
smaller  lens  and  carry  the  screw  cell. 

The  above  mentioned  adjustable  holders  can  also  be  made  for 
filters  in  “ A ” glass,  but  they  are  not  to  be  recommended  for 
these  heavy  filters. 

Sets  of  tricolour  filters,  if  small,  are  best  fitted  in  a repeating 
back,  and  used  with  a dark  slide  carrying  long  plates;  the  three 
negatives  being  taken  side  by  side  on  the  same  plate.  Where 
larger  sizes  are  required,  or  where  it  is  not  desired  to  use  plates 
of  special  sizes,  the  filters  may  be  fitted  in  a frame  which  can 
slide  behind  the  lens  through  an  outside  protecting  cover  screwed 
to  the  lens  panel,  thus  changing  the  filters  somewhat  in  the 

1 10 


THE  FITTING  OF  FILTERS 


manner  in  which  lantern  slides  are  changed  by  a lantern  slide 
carrier.  Although  this  method  is  rather  slow  in  changing — as  the 
dark  slide  must  be  changed  as  well  as  the  filter — it  is  in  other 
respeCts  a satisfactory  fitting  (fig.  63). 

It  is  also  possible  to  make  a holder,  either  of  metal  or  of 
wood,  to  fit  on  to  the  lens,  into  which  the  three  filters  can  be 
slipped  in  turn;  this  holder  can  also  be  used  for  any  other  filters 
in  addition  to  the  tricolour  set. 

It  is  not  advisable  to  use  screw  cells  for  tricolour  filters,  as 
the  changing  of  the  cell  is  almost  sure  to  shake  the  camera,  and 
involves  much  loss  of  time.  For  process  cameras  special  holders 
for  illters  are  advisable,  and  one  holder  in  which  each  filter  is 
put  in  place  separately  is  probably  the  most  satisfactory.  In  such 
work  great  care  is  necessary  that  each  filter  is  put  in  the  same 
place,  and  exadtly  square  to  the  lens.  One  way  up  may  be  better 
than  another,  and  all  Wratten  three-colour  filters  are  marked 
to  show  the  best  way  to  insert  the  filter  into  the  holder. 


1 1 1 


CHAPTER  XIII 


THE  CARE  OF  FILTERS 

IN  its  simplest  form  (gelatine  film)  a filter  requires  a consider- 
able amount  of  care  in  handling.  The  safest  way  is  to  place 
it  between  the  combinations  of  a lens  and  leave  it  there.  If  it  be 
used  in  front  of  or  behind  the  lens  in  any  form  of  carrier  it 
should  be  removed  after  use  and  placed,  in  clean  paper,  between 
the  leaves  of  a book,  where  it  will  keep  flat  and  dry.  Moisture 
tends  to  cloud  gelatine  film  filters.  The  fingers  are  almost  in- 
variably moist  and,  to  a certain  extent,  greasy,  hence  in  handling 
gelatine  films  care  should  be  exercised  to  hold  them  by  the  ex- 
treme corner,  if  the  filters  be  square,  or,  better  still,  by  the  edges 
only. 

If  it  be  necessary  to  cut  the  film  it  should  be  placed  between 
two  clean  pieces  of  fairly  stiff  paper,  note-paper  for  instance,  and 
cut  with  a sharp  pair  of  scissors.  A knife  can  also  be  used  if  the 
film  is  firmly  held  between  two  pieces  of  glass  and  trimmed  round, 
taking  care  to  cut  and  not  simply  to  pull  with  the  knife,  as  the 
film  very  easily  splinters  and  cracks. 

A film  filter  may  be  dusted  with  a piece  of  soft  silk,  or  per- 
haps a better  plan  is  to  use  the  edge  of  a piece  of  very  fine  tissue 
paper.  The  latter  will  not  scratch  if  used  carefully,  and  does 
not  leave  any  fluff  on  the  film. 

Cemented  filters  should  be  treated  with  care  equal  to  that 
accorded  to  lenses,  they  should  be  kept  in  their  cases  and  on  no 
account  allowed  to  get  damp  or  dirty.  Filters  are  clean  when 
first  sent  out  by  the  makers,  and  with  reasonable  care  in  handling 
they  should  never  become  so  dirty  as  to  require  other  cleaning 
than  can  be  given  by  breathing  upon  them,  and  polishing  with 
a piece  of  silk  or  tissue  paper.  A filter  should  never  be  washed 
with  water,  under  any  circumstances,  because  if  water  comes 
into  contact  with  the  gelatine  at  the  edges  of  the  filter  it  will 
cause  it  to  swell  and  so  separate  the  glasses,  causing  air  to  run  in 
between  the  gelatine  and  the  glass.  Even  if  the  swelling  does 
not  cause  air  to  run  in  in  this  manner,  the  filter  will  be  strained 
and  the  definition  spoiled. 


THE  CARE  OF  FILTERS 


If  for  any  reason  the  filter  gets  so  dirty  that  it  cannot  he 
cleaned  bv  simple  rubbing,  after  breathing  on  it,  a piece  of  fine 
tissue  paper  should  be  damped  with  methylated  spirit  and  gently 
rubbed  over  the  surface  of  the  filter.  Care  must  be  taken  that 
the  tissue  paper  is  not  wet  enough  for  the  spirit  to  run  out  and 
spread  over  the  edge  of  the  filter,  as  methylated  spirit  is  a solvent 
of  the  balsam  with  which  filters  are  cemented,  and  will  soften  it 
so  that  air  may  run  in.  Before  attempting  to  clean  a filter  at  all 
it  is  well  to  make  sure  that  both  the  surface  of  the  glass  and  the 
material  are  entirely  free  from  grit,  which  will  scratch  the  glass. 
If  the  glass  becomes  badly  scratched  the  only  thing  to  do  is  to 
recement  the  filter  with  the  scratch  inside,  which  can  be  done 
i(U  the  case  of  Hats,  although  this  involves  a considerable  delay 
while  the  recemented  filter  is  drying. 

In  addition  to  moisture,  undue  heat  is  dangerous  to  filters,  as 
it  softens  the  balsam  and  causes  the  gelatine  to  contract,  so  that 
filters  should  always  be  protected  from  heat,  as  far  as  possible. 

The  dves  used  for  most  filters  are  quite  stable  to  light,  a 
table  showing  the  stability  to  light  of  all  the  Wratten  filters 
being  published  in  u Wratten  Light  Filters.” 

The  “ K ” filters  are  particularly  stable,  and  no  fear  of  fading 
need  be  felt.  Filters,  however,  should  always  be  kept  in  their 
cases  when  not  required  for  use;  tricolour  filters,  especially, 
should  be  put  back  into  the  cases  as  they  are  taken  from  the 
camera  in  turn,  they  are  then  quite  safe  and  can  always  be  found 
when  required. 

The  Aesculine  filters,  used  for  the  removal  of  ultra-violet 
light  in  photographing  drawings  which  contain  Chinese  white, 
must  be  protected  from  light,  to  which  they  are  unstable,  going 
brown  when  exposed  to  it  for  any  length  of  time. 


INDEX 


“ A ” FILTER,  38. 

Aberrations  in  relation  to  focal 
length  of  lens,  97. 

Abney,  Sir  W.  A.,  92. 

'Absorption,  10,  39. 
of  dyed  wools,  17-18. 
of  erythrosine,  12. 
of  Gentian  violet,  12. 
measurements  of,  16-17. 
of  natural  colours,  14. 
partial,  12. 
of  printing  inks,  17. 
sharp  and  gradual,  13. 
ultra  violet,  32. 

A&inometer,  use  of  in  high  alti- 
tudes, 72. 

Aesculine  filter,  the,  113. 

Alpine  photography,  69,  75. 

Angstrom  ^inits,  9. 

Anti-orthochromatic  filter,  33. 

Artificial  light,  photography  with, 
29,  42,  67. 

Astigmatism  in  filters,  ioi. 

Atmosphere  in  photographs,  33,  37. 

Autochrome  plate,  the,  87. 


Cloud  photography,  38,  72,  76. 
Colour,  associated  with  wave 
length,  8. 

-box  of  Clerk  Maxwell,  82. 
complementary,  n. 
chart,  25,  30. 

chart,  photographs  of,  26,29,  44- 
contrast,  rendering  of,  47. 
contrast  and  tone  contrast  com- 
pared, 47. 
definition  of,  10, 
means  absorption,  10. 
nature  of,  7. 

perception,  variation  with  in- 
tensity, 20. 

Colours,  with  equal  luminosity,  ren- 
dering of,  47. 
under  mercury  vapour,  10. 
Commercial  photography,  54. 
Complementary  colours,  1 1. 
Contrast  filters,  42. 

possible  in  halt-tone  work,  78. 
Copying  sepia  and  brown  prints, 
52* 

Curves  of  plate  sensitiveness,  21,22. 


Bathing  plates  to  sensitize,  22-23. 
Blue  filter,  the,  91. 

Blue  prints,  to  copy,  54. 

Brown  glass  filters,  32. 

Bull,  Mr.  A.  J.,  89,  95. 

Callier,  Andre,  69. 

on  plates  for  landscape  photo- 
graphy, 72. 

Catalogue  illustrations,  54. 
“Chemical”  or  ultra-violet  rays, 
27-28. 

Chinese  white,  absorption  of  ultra- 
violet, 33. 

Chromo-lithography,  77. 

Clerk  Maxwell,  82. 

Clothing  in  portraiture,  66. 


Daffodil,  photography  of,  24. 
Daylight  photography,  29-30. 
Defers  in  colour  rendering  due  to 
inks,  94. 

Distance,  photography  of,  33,  74. 
Ducos  du  Hauron,  86. 

Dyed  wool  absorptions,  17-18. 


Effect  of  exposing  under  three  tri- 
colour filters.  49. 

Eosine,  21,  50. 

Erythrosine,  21,  39. 

Estate  photography,  56. 

Exposure,  37. 

in  portraiture,  66. 
with  filters,  31. 

Eye,  perception  limits  of,  19. 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


Eye,  function  of  the  rods  and  cones, 
zo. 

safelights  considered  in  relation 
to,  21. 

FaCtors  for  Wratten  contrast  filters, 

43- 

for  contrast  filters,  43. 

Filter  holders,  1 1 1. 

adjustable,  110. 

Filters,  Aesculine,  113. 
anti-orthochromatic,  33. 
astigmatism  in,  101. 
brown  glass,  32. 
care  of,  1 1 2. 
cells  for,  106. 
cemented,  112. 
contrast,  42. 

criterion  of  efficiency  of,  33. 
drying  after  cementing,  100. 
effect  of  strain  on,  100. 
effeCt  on  focus  of,  103. 
efficiency  of,  31. 
errors  in,  98. 
tests  for,  97. 
film,  99,  1 12. 
fitting  of,  105. 
for  microscopical  work,  98. 
in  flats,  information  necessary 
when  ordering,  109. 
green  glass,  32. 

how  to  determine  size  required, 
108. 

ideal,  31. 

“K,”  33,  34. 

measurements  for  fitting,  107. 
orthochromatic  or  isochromatic, 

30. 

orthochromatic,  need  for,  24. 
optical  properties  of,  97. 
necessary  in  subtraCtive  work,  90. 
in  portraiture,  66. 
registration  with,  104. 
sharp-cut,  39. 
stability  of,  1 1 3. 
three-colour,  48. 
two  used  together,  42. 
varieties  of,  98. 

Flashlight,  68. 

F lat  and  foggy  negatives,  cause  of,  7 5. 
Flesh  tints,  61. 

Flower  photography,  25,  35. 
Frontispiece,  photographs  of,  44. 
Furniture  photography,  58. 

I 16 


“ G filter,  37,  43. 

Gentian  violet,  absorption  of,  12. 
Gradation  of  negatives  in  three- 
colour  work,  92. 

Green  colour,  1 1 . 

Green  filter,  the,  90. 

Green  glass  filters,  32. 

Greens,  reproduction  of,  95. 
theoretical  and  aCtual,  1 5. 

Hair  in  portraiture,  64. 

Half-watt  lamps,  68. 

Haze  and  atmosphere,  37. 

“ High  light  ” negatives,  78. 

Houses  and  buildings,  photography 
of,  56. 

Hue  in  colour  reproduction,  83. 

Incandescent  gas,  42. 

Infra-red,  photography  in,  76. 

Joly,  Prof.,  86,  87. 

Jones,  Mr.  Chapman,  92. 

“K”  filters,  33,  34,  35. 

efficiency  of,  34. 

Koenig,  Dr.  E.,  17,  89. 

Landscape  photography,  35,  37,  69. 
Lens  hood,  need  for,  74. 

Light,  analysis  of,  8-9. 
filters,  10. 
frequencies,  8. 
nature  of,  7. 

and  sound,  analogy  between,  7. 
theories  of,  7. 
velocity  of,  8. 
waves,  8. 

Lumiere  Autochrome  plate,  87. 
Luminosities,  correCt  rendering  of, 
24. 

Luminosity  curve,  16. 

of  speCtrum  on  ordinary  and  col- 
our sensitive  plates,  27. 
Luminosity  of  colours,  16. 

“ M ” filters,  45. 

“ M ” plates,  45. 

Mahogany,  to  photograph,  58. 
Maxwell,  Clerk,  82. 

Measurements  for  filter  fitting,  107. 
Mees,  Dr.  C.  E.  K.,  Preface,  16, 
88,  91. 

Mercury  vapour  lamp,  10,  63. 


INDEX 


Misconceptions  of  tone  values  of 
colours,  48. 

Mist,  effeCt  of  in  photography,  73. 
its  causes,  73. 
scattering  of,  73-7+. 
scattering  of  light  by,  33. 
Multiplying  fa  ft  or  for  filters,  31. 

Newton,  Mr.  A.  J.,  89. 

Night  photography,  68. 

Orthochromatic  filters,  efficiency  of, 
31* 

Orthochromatic  plates,  40. 
Over-correftion,  33,  35. 

Photochromoscope,  the,  84. 
Photographs,  tinting,  66. 
Photography,  landscape,  35. 
of  flowers,  24-25,  35. 
by  night,  68. 

Photomicrography,”  58. 

Pifture  copying,  48. 

Plate  sensitiveness,  curves  of,  21. 
Plates,  orthochromatic,  40. 

“ red-blind,”  34. 

Portraiture,  61. 

artificial  light  for,  67. 
clothing  in,  66. 
exposure  in,  66. 
filters  in,  66. 
hair  in,  64. 

in  ordinary  rooms,  67. 

with  mercury  vapour  lamps,  68. 

through  red  filter,  64. 

Postage  stamps,  to  photograph,  56. 
Posters,  to  photograph,  56. 
Principles  in  three-colour  photo- 
graphy, 50. 

Printing  ink  absorptions,  17. 
inks,  ideal  and  real,  93. 
colours  for  subtraftive  process, 
88. 

colours,  seleftion  of,  93. 
Projeftion,  three  colour,  84. 
Purkinje’s  phenomenon,  20. 

Red  and  black,  to  reproduce,  80. 
Red  silver  prints,  to  photograph,  60. 
Relief  processes,  78. 

Reproduction,  photography  of  col- 
oured objefts  for,  77. 

“ Reproduction  Work  with  Dry 
Plates,”  78-79. 


Retouching,  how  to  avoid,  61. 

Rules  for  photographing  colour,  15. 

Safelights  considered  in  relation  to 
eye  sensitiveness,  21. 

Screen  plate  colour  photography,  86. 
Sensation  curves,  three-colour,  82. 
Sensitiveness  conferred  by  dyeing, 
21. 

distribution  of  in  plates,  23. 
of  eye  and  photographic  plate 
compared,  19. 

of  plates  to  artificial  lights,  67. 
of  plates  to  various  colours,  21. 
ratio  of  to  different  colours,  22. 
Sensitizing  plates  by  bathing,  22- 

. 23*. 

Sepia  prints,  to  copy,  52. 

Sheppard,  Dr.  S.  E.,  91. 

Skin  texture,  to  photograph,  63. 

Sky  in  landscape,  69. 

Speftroscope,  the,  9. 

Speftrum,  the,  9. 

represented  in  monochrome,  19. 
reproduction  of,  83. 

Stains,  to  photograph,  52. 

Stenger,  Prof.  E.,  92. 

Straining  a filter,  effeft  of,  100. 
Subtraftive  process  of  colour-photo- 
graphy, 88. 


Technical  photography,  54. 
Telephotography,  37,  74. 

Tinting  photographs,  66. 
Three-colour  half-tone  by  direft 
method,  78. 
photography,  82. 
projection,  filters  for,  84-85. 

Tone  contrast,  47. 

Tricolour  filters,  48. 

Two-colour  reproduction,  80. 
Typewriting,  to  photograph,  54. 

Ultra-violet,  21. 

or  “chemical  rays,”  27-28,  29, 
3*»  33»  34>  35>  36,  39- 

Violet,  theoretical  and  aftual,  15. 
Violets,  reproduction  of,  96. 

Vogel’s  discovery,  21. 


7 


PHOTOGRAPHY  OF  COLOURED  OBJECTS 


Wall,  Mr.  E.  J.,  89. 

Wood,  Prof.  R.  W.,  33,  76. 
Wratten  copyboard  chart,  93. 
ink  control,  96. 
ink  tester,  96. 
green  filter,  91. 

panchromatic  plate,  23,  28,  30, 
36,  39,  40,  41,  47. 


Wratten  process  panchromatic  plate, 
54- 

Wrinkles,  exaggeration  of  in  ordin- 
ary portraiture,  63. 

Yellow,  its  composition,  1 1. 
Yellowed  prints,  52. 


3 3125  00002  9278 


